PDL::Graphics::Simple - Simple backend-independent plotting for PDL


 # Simple interface - throw plots up on-screen, ASAP
 use PDL::Graphics::Simple;
 imag $a;                     # Display an image PDL
 imag $a, 0, 300;             # Display with color range
 line $rrr, $fit;             # Plot a line

 points $rr, $sec;            # Plot points
 hold;                        # Hold graphics so subsequent calls overplot
 line $rrr, $fit;             # Overplot a line in a contrasting color
 release;                     # Release graphics

 # Object interface - simple plotting, to file or screen
 $w = pgswin( size=>[8,4], multi=>[2,2] ); # 2x2 plot grid on an 8"x4" window
 $w = pgswin( size=>[1000,1000,'px'], output=>'plot.png' ); # output to a PNG

 $w->plot( with=>'points', $rr, $sec, with=>'line', $rrr, $fit,
           {title=>"Points and fit", xlabel=>"Abscissa", ylabel=>"Ordinate"});


PDL can plot through a plethora of external plotting modules. Each module tends to be less widely available than Perl itself, and to require an additional step or two to install. For simple applications ("throw up an image on the screen", or "plot a curve") it is useful to have a subset of all plotting capability available in a backend-independent layer. PDL::Graphics::Simple provides that capability.

PDL::Graphics::Simple implements all the functionality used in the PDL::Book examples, with identical syntax. It also generalizes that syntax - you can use ::Simple graphics, with slight syntactical differences, in the same manner that you would use any of the engine modules. See the Examples below for details.

The plot you get will always be what you asked for, regardless of which plotting engine you have installed on your system.

Only a small subset of PDL's complete graphics functionality is supported -- each individual plotting module has unique advantages and functionality that are beyond what PDL::Graphics::Simple can do. Only 2-D plotting is supported. For 3-D plotting, use PDL::Graphics::Gnuplot or PDL::Graphics::Trid directly.

When plotting to a file, the file output is not guaranteed to be present until the plot object is destroyed (e.g. by being undefed or going out of scope).


PDL::Graphics::Simple currently supports most of the planned functionality. It is being released as a beta test to determine if it meets users' needs and gain feedback on the API -- so please give feedback!


PDL::Graphics::Simple includes support for the following graphics engines. Additional driver modules can be loaded dynamically; see register, below. Each of the engines has unique capabilities and flavor that are not captured in PDL::Graphics::Simple - you are encouraged to look at the individual modules for more capability!

  • Gnuplot (via PDL::Graphics::Gnuplot)

    Gnuplot is an extremely richly featured plotting package that offers markup, rich text control, RGB color, and 2-D and 3-D plotting. Its output is publication quality. It is supported on POSIX systems, MacOS, and Microsoft Windows, and is available from most package managers.

  • PGPLOT (via PDL::Graphics::PGPLOT::Window)

    PGPLOT is venerable and nearly as fully featured as Gnuplot for 2-D plotting. It lacks RGB color output. It does have rich text control, but uses simple plotter fonts that are generated internally. It is supported on MacOS and POSIX, but is not as widely available as Gnuplot.

  • PLplot (via PDL::Graphics::PLplot)

    PLplot is a moderately full featured plotting package that generates publication quality output with a simple high-level interface. It is supported on MacOS and POSIX.

  • Prima (via PDL::Graphics::Prima)

    Prima is based around a widget paradigm that enables complex interaction with data in real-time, and it is highly optimized for that application. It is not as mature as the other platforms, particularly for static plot generation to files. This means that PDL::Graphics::Simple does not play to its considerable strengths, although Prima is serviceable and fast in this application. Please run the Prima demo in the perldl shell for a better sample of Prima's capabilities.


PDL::Graphics::Simple can be called using plot-atomic or curve-atomic plotting styles, using a pidgin form of calls to any of the main modules. The examples are divided into Book-like (very simple), PGPLOT-like (curve-atomic), and Gnuplot-like (plot-atomic) cases.

There are three main styles of interaction with plot objects that PDL::Graphics::Simple supports, reflective of the pre-existing modules' styles of interaction. You can mix-and-match them to match your particular needs and coding style. Here are examples showing convenient ways to call the code.

First steps (non-object-oriented)

For the very simplest actions there are non-object-oriented shortcuts. Here are some examples of simple tasks, including axis labels and plot titles. These non-object-oriented shortcuts are useful for display with the default window size. They make use of a package-global plot object.

The non-object interface will keep using the last plot engine you used successfully. On first start, you can specify an engine with the environment variable PDL_SIMPLE_ENGINE. If that one isn't working, or if you didn't specify one, all known engines are tried in alphabetical order until one works.

  • Load module and create line plots

     use PDL::Graphics::Simple;
     $x = xvals(51)/5;
     $y = $x**3;
     line( $x, $y );
     line( $x, $y, {title=>"My plot", ylabel=> "Ordinate", xlabel=>"Abscissa"} );
  • Bin plots

     bins($y, {title=>"Bin plot", xl=>"Bin number", yl=>"Count"} );
  • Point plots

     points($y, {title=>"Points plot"});
  • Logarithmic scaling

     line( $y, { log=>'y' } );    # semilog
     line( $y, { log=>'xy' } );   # log-log
  • Image display

     $im = 10 * sin(rvals(101,101)) / (10 + rvals(101,101));
     imag $im;          # Display image
     imag $im, 0, 1;    # Set lower/upper color range
  • Overlays

     points($x, $y, {logx=>1});
     line($x, sqrt($y)*10);
  • Justify aspect ratio

     imag $im, {justify=>1}
     points($x, $y, {justify=>1});
  • Erase/delete the plot window


Simple object-oriented plotting

More functionality is accessible through direct use of the PDL::Graphics::Simple object. You can set plot size, direct plots to files, and set up multi-panel plots.

The constructor accepts window configuration options that set the plotting environment, including size, driving plot engine, output, and multiple panels in a single window.

For interactive/display plots, the plot is rendered immediately, and lasts until the object is destroyed. For file plots, the file is not guaranteed to exist and be correct until the object is destroyed.

The basic plotting method is plot. plot accepts a collection of arguments that describe one or more "curves" (or datasets) to plot, followed by an optional plot option hash that affects the entire plot. Overplotting is implemented via plot option, via a held/released state (as in PGPLOT), and via a convenience method oplot that causes the current plot to be overplotted on the previous one.

Plot style (line/points/bins/etc.) is selected via the with curve option. Several convenience methods exist to create plots in the various styles.

  • Load module and create basic objects

     use PDL::Graphics::Simple;
     $x = xvals(51)/5;
     $y = $x**3;
     $win = pgswin();                       # plot to a default-shape window
     $win = pgswin( size=>[4,3] );          # size is given in inches by default
     $win = pgswin( size=>[10,5,'cm'] );    # You can feed in other units too
     $win = pgswin( out=>'' );       # Plot to a file (type is via suffix)
     $win = pgswin( engine=>'gnuplot' );    # Pick a particular plotting engine
     $win = pgswin( multi=>[2,2] );         # Set up for a 2x2 4-panel plot
  • Simple plots with plot

     $win->plot( with=>'line', $x, $y, {title=>"Simple line plot"} );
     $win->plot( with=>'errorbars', $x, $y, sqrt($y), {title=>"Error bars"} );
     $win->plot( with=>'circles', $x, $y, sin($x)**2 );
  • Plot overlays

     # All at once
     $win->plot( with=>'line', $x, $y,   with=>'circles', $x, $y/2, sqrt($y)  );
     # Using oplot (IDL-style; PLplot-style)
     $win->plot(  with=>'line', $x, $y );
     $win->oplot( with=>'circles', $x, $y/2, sqrt($y) );
     # Using object state (PGPLOT-style)
     $win->line(  $x, $y );
     $win->circles( $x, $y/2, sqrt($y) );




show lists the supported engines and a one-line synopsis of each.

pgswin - exported constructor

 $w = pgswin( %opts );

pgswin is a constructor that is exported by default into the using package. Calling pgswin(%opts) is exactly the same as calling PDL::Graphics::Simple->new(%opts).


 $w = new PDL::Graphics::Simple( %opts );

new is the main constructor for PDL::Graphics::Simple. It accepts a list of options about the type of window you want:


If specified, this must be one of the supported plotting engines. You can use a module name or the shortened name. If you don't give one, the constructor will try the last one you used, or else scan through existing modules and pick one that seems to work. It will first check the environment variable PDL_SIMPLE_ENGINE, then search through all the known engines in alphabetical order until it finds one that seems to work on your system.


This is a window size as an ARRAY ref containing [width, height, units]. If no units are specified, the default is "inches". Accepted units are "in","pt","px","mm", and "cm". The conversion used for pixels is 100 px/inch.


This describes the kind of plot to create, and should be either "file" or "interactive" - though only the leading character is checked. If you don't specify either type or output (below), the default is "interactive". If you specify only output, the default is "file".

For PGPLOT, if the type is "interactive", the environment variable PGPLOT_DEV is set (eg /NULL), that will be used as the output device.


This should be a window number or name for interactive plots, or a file name for file plots. The default file name is "plot.png" in the current working directory. Individual plotting modules all support at least '.png', '.pdf', and '.ps' -- via format conversion if necessary. Most other standard file types are supported but are not guaranteed to work.


This enables plotting multiple plots on a single screen. You feed in a single array ref containing (nx, ny). Subsequent calls to plot send graphics to subsequent locations on the window. The ordering is always horizontal first, and left-to-right, top-to-bottom.


 $w = new PDL::Graphics::Simple ( %opts );

plot plots zero or more traces of data on a graph. It accepts two kinds of options: plot options that affect the whole plot, and curve options that affect each curve. The arguments are divided into "curve blocks", each of which contains a curve options hash followed by data.

If the last argument is a hash ref, it is always treated as plot options. If the first and second arguments are both hash refs, then the first argument is treated as plot options and the second as curve options for the first curve block.

Plot options:


If this is set, then the plot overplots a previous plot.


If this is set, it is a title for the plot as a whole.


If this is set, it is a title for the X axis.


If this is set, it is a title for the Y axis.


If this is set, it is a two-element ARRAY ref containing a range for the X axis. If it is clear, the axis is autoscaled.


If this is set, it is a two-element ARRAY ref containing a range for the Y axis. If it is clear, the axis is autoscaled.


This should be empty, "x", "y", or "xy" (case and order insensitive). Named axes are scaled logarithmically.


If this is set, it is a two-element ARRAY ref containing a range for color values, full black to full white. If it is clear, the engine or plot module is responsible for setting the range.


If this is set, then image plots get a scientific colorbar on the right side of the plot. (You can also say "colorbar", "colorbox", or "cb" if you're more familiar with Gnuplot).


If this is set to a true value, then the screen aspect ratio is adjusted to keep the Y axis and X axis scales equal -- so circles appear circular, and squares appear square.


The "legend" plot option is intended for full support but it is currently experimental: it is not fully implemented in all the engines, and implementation is more variable than one would like in the engines that do support it.

This controls whether and where a plot legend should be placed. If you set it, you supply a combination of 't','b','c','l', and 'r': indicating top, bottom, center, left, right position for the plot legend. For example, 'tl' for top left, 'tc' for center top, 'c' or 'cc' for dead center. If left unset, no legend will be plotted. If you set it but don't specify a position (or part of one), it defaults to top and left.

If you supply even one 'key' curve option in the curves, legend defaults to the value 'tl' if it isn't specified.

Curve options:


This names the type of curve to be plotted. See below for supported curve types.


This gives a name for the following curve, to be placed in a master plot legend. If you don't specify a name but do call for a legend, the curve will be named with the plot type and number (e.g. "line 3" or "points 4").


This lets you specify the width of the line, as a multiplier on the standard width the engine uses. That lets you pick normal-width or extra-bold lines for any given curve. The option takes a single positive natural number.


You can specify the line style in a very limited way -- as a style number supported by the backend. The styles are generally defined by a mix of color and dash pattern, but the particular color and dash pattern depend on the engine in use. The first 30 styles are guaranteed to be distinguishable. This is useful to produce, e.g., multiple traces with the same style.

Curve types supported


This is a simple point plot. It takes 1 or 2 columns of data.


This is a simple line plot. It takes 1 or 2 columns of data.


Stepwise line plot, with the steps centered on each X value. 1 or 2 columns.


Simple points-with-errorbar plot, with centered errorbars. It takes 2 or 3 columns, and the last column is the absolute size of the errorbar (which is centered on the data point).


Simple points-with-errorbar plot, with asymmetric errorbars. It takes 3 or 4 columns, and the last two columns are the absolute low and high values of the errorbar around each point (specified relative to the origin, not relative to the data point value).


Plot unfilled circles. Requires 2 or 3 columns of data; the last column is the radius of each circle. The circles are circular in scientific coordinates, not necessarily in screen coordinates (unless you specify the "justify" plot option).


This is a monochrome or RGB image. It takes a 2-D or 3-D array of values, as (width x height x color-index). Images are displayed in a sepiatone color scale that enhances contrast and preserves intensity when converted to grayscale. If you use the convenience routines (image or imag), the "justify" plot option defaults to 1 -- so the image will be displayed with square pixel aspect. If you use plot(with=>'image' ...), "justify" defaults to 0 and you will have to set it if you want square pixels.

For RGB images, the numerical values need to be in the range 0-255, as they are interpreted as 8 bits per plane colour values. E.g.:

  $w = pgswin(); # plot to a default-shape window
  $w->image( pdl(xvals(9,9),yvals(9,9),rvals(9,9))*20 );

  # or, from an image on disk:
  $image_data = rpic( 'my-image.png' )->mv(0,-1); # need RGB 3-dim last
  $w->image( $image_data );

This places text annotations on the plot. It requires three input arguments: the X and Y location(s) as PDLs, and the label(s) as a list ref. The labels are normally left-justified, but you can explicitly set the alignment for each one by beginning the label with "<" for left "|" for center, and ">" for right justification, or a single " " to denote default justification (left).


 $w = new PDL::Graphics::Simple ( %opts );

oplot is a convenience interface. It is exactly equivalent to plot except it sets the plot option oplot, so that the plot will be overlain on the previous one.

line, points, image, imag

 # Object-oriented convenience
 $w = new PDL::Graphics::Simple ( % opts );

 # Very Lazy Convenience
 $a = xvals(50);
 lines $a;
 $im = sin(rvals(100,100)/3);
 imag $im;
 imag $im, 0, 1, {title=>"Bullseye?", j=>1};

line, points, and image are convenience interfaces. They are exactly equivalent to plot except that they set the default "with" curve option to the appropriate plot type.

imag is even more DWIMMy for PGPLOT users or PDL Book readers: it accepts up to three non-hash arguments at the start of the argument list. The second and third are taken to be values for the crange plot option.


 use PDL::Graphics::Simple qw/erase hold release/;
 line xvals(10), xvals(10)**2 ;
 sleep 5;

erase removes a global plot window. It should not be called as a method. To remove a plot window contained in a variable, undefine it.


 use PDL::Graphics::Simple;
 line xvals(10);
 line xvals(10)**0.5;

Causes subsequent plots to be overplotted on any existing one. Called as a function with no arguments, hold applies to the global object. Called as an object method, it applies to the object.


 use PDL::Graphics::Simple;
 line xvals(10);
 line xvals(10)**0.5;
 line xvals(10)**0.5;

Releases a hold placed by hold.


 PDL::Graphics::Simple::register( $module_name );

This is the registration mechanism for new driver methods for PDL::Graphics::Simple. Compliant drivers should announce themselves at compile time by calling register. When they do that, they should have already defined a package global hash ref, $mod, containing the following keys:


This is the short name of the engine, by which users refer to it colloquially.


This is the fully qualified package name of the module itself.


This is the fully qualified package name of the Perl API for the graphics engine.


This is a brief string describing the backend


This is a one-period version number of PDL::Graphics::Simple against which the module has been tested. A warning will be thrown if the version isn't the same as $PDL::Graphics::Simple::VERSION.


PDL::Graphics::Simple defines an object that represents a plotting window/interface. When you construct the object, you can either specify a backend or allow PDL::Graphics::Simple to find a backend that seems to work on your system. Subsequent plotting commands are translated and passed through to that working plotting module.

PDL::Graphics::Simple calls are dispatched in a two-step process. The main module curries the arguments, parsing them into a regularized form and carrying out DWIM optimizations. The regularized arguments are passed to subclasses that translate them into the APIs of their respective plot engines. The subclasses are very simple and implement only a few methods, outlined below. They are intended only to be called by the PDL::Graphics::Simple driver, which limits the need for argument processing, currying, and parsing. The subclasses are thus responsible only for converting the regularized parameters to plot calls in the form expected by their corresponding plot modules.

PDL::Graphics::Simple works through a call-and-dispatch system rather than taking full advantage of inheritance. That is for two reasons: (1) it makes central control mildly easier going forward, since calls are dispatched through the main module; and (2) it makes the non-object-oriented interface easier to implement since the main interface modules are in one place and can access the global object easily.

Interface subclass methods

Each interface module supports the following methods:


check attempts to load the relevant engine module and test that it is working. In addition to returning a boolean value indicating success if true, it registers its success or failure in the main $mods hash, under the "ok" flag. If there is a failure that generates an error message, the error is logged under the "msg" flag.

check accepts one parameter, "force". If it is missing or false, and "ok" is defined, check just echoes the prior result. If it is true, then check actually checks the status regardless of the "ok" flag.


new creates and returns an appropriate plot object, or dies on failure.

Each new method should accept the following options, defined as in the description for PDL::Graphics::Simple::new (above). There is no need to set default values as all arguments should be set to reasonable values by the superclass.

For file output, the method should autodetect file type by dot-suffix. At least ".png" and ".ps" should be supported.

Required options: size, type, output, multi.


plot generates a plot. It should accept a standardized collection of options as generated by the PDL::Graphics::Simple plot method: standard plot options as a hash ref, followed by a list of curve blocks. It should render either a full-sized plot that fills the plot window or, if the object multi option was set on construction, the current subwindow. For interactive plot types it should act as an atomic plot operation, displaying the complete plot. For file plot types the atomicity is not well defined, since multiplot grids may be problematic, but the plot should be closed as soon as practical.

The plot options hash contains the plot options listed under plot, above, plus one additional flag - oplot - that indicates the new data is to be overplotted on top of whatever is already present in the plotting window. All options are present in the hash. The title, xlabel, ylabel, and legend options default to undef, which indicates the corresponding plot feature should not be rendered. The oplot, xrange, yrange, crange, wedge, and justify parameters are always both present and defined.

If the oplot plot option is set, then the plot should be overlain on a previous plot - otherwise the module should display a fresh plot.

Each curve block consists of an ARRAY ref with a hash in the 0 element and all required data in the following elements, one PDL per (ordinate/abscissa). For 1-D plot types (like points and lines) the PDLs must be 1D. For image plot types the lone PDL must be 2D (monochrome) or 3D(RGB).

The hash in the curve block contains the curve options for that particular curve. They are all set to have reasonable default values. The values passed in are with and key. If the legend option is undefined, then the curve should not be placed into a plot legend (if present). The with option will be one of points, lines, bins, errorbars, limitbars, circles image, or labels.


Deal with legend generation. In particular: adding legends with multi-call protocols is awkward and leads to many edge cases in the internal protocol. This needs more thought.



Fix tests for smoker compatibility


Include Prima support



Craig DeForest, <>


Copyright 2013 Craig DeForest

This program is free software; you can redistribute it and/or modify it under the terms of either: the Gnu General Public License v2 as published by the Free Software Foundation; or the Perl Artistic License included with the Perl language.

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