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Ruby Gnuplot – How To

Rubyforge Project
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ChangeLog
Authors
License

History and Background

Gnuplot is a program that has a rich language for the generation of plots. It has a unique place in academia as it was one of the first freely available programs for plot generation. I started using gnuplot over 10 years ago while pursuing my Master’s degree in Physics and have been using it actively ever since.

Version 0.9

My first attempt at a Ruby interface to gnuplot was an object interface
encapsulating gnuplot language. This was taken directly from the Python
gnuplot interface. In spite of my being very familiar with Gnuplot and
Ruby and being the author of the RGnuplot package, I found it
non-intuitive to use the RGnuplot package. I found myself constantly
looking at the code to figure out what I needed to do.
This was not sufficient and did not sit well.

Version 1.0

The second attempt at a Ruby interface was to do absolutely nothing but
use Ruby’s built in string manipulation methods. This meant that I
could simply use my knowledge of Gnuplot without having to worry about
objects.
While in some ways an improvement over Version 0.9, it still did not sit
well with me.

Version 2.0

After attending RubyConf 2004 I was inspired by Rich Kilmer’s use of
Ruby to implement domain specific languages. That is the current
implementation of Gnuplot and quite probably the one that I’ll stick
with for some time. This version combines the direct mapping of the
gnuplot language without wrapping with the ruby syntax and mechanism of
adding methods to existing classes to interface Ruby objects with
gnuplot.

Setup

Version 2.2

If the ‘gnuplot’ command is in your path then there is no required setup. If the gnuplot executable for your system is called something other than simply ‘gnuplot’ then set the RB_GNUPLOT environment variable to the name of the executable. This must either be a full path to the gnuplot command or an executable filename that exists in your PATH environment variable.

Ruby Gnuplot Concepts

Gnuplot has a very simple conceptual model. Calls to Set are made to set parameters and either Plot or Splot is called to generate the actual plot. The dataset to be plotted can be specified in a number of ways, contained in a seperate file, generated from a function, read from standard input, or read immediately after the plot command. The object model for the Ruby gnuplot wrapper directly mimics this layout and flow. The following are the standard steps for generating a plot:

Instantiate a Plot or Splot object and set parameters by gnuplot variable name.
p. Instantiate DataSet objects and attach Ruby objects containing
the data to be plotted to the DataSet. Attach properties that modify
the plot command using the modifier name.

Send the Plot/Splot object to a Gnuplot instance for
plotting.

The Version 2.0 interface makes very heavy use of blocks leading to very readable code.

Gnuplot.open

Instantiates a new Gnuplot process. The path to the executable is
determined on a Unix or MacOSX system using the which command. Windows
users, I have no idea what to do.
If a block is given to the function the opened process is passed into
the block. This mimics the most common usage of the File.open method.

Plot.new

SPlot.new

Create a new Plot or Splot object. DataSets are attached to the object
to specify the data and its properties.
If a block is given to the function, the plot object is passed into the
block.

DataSet.new

Associates a Ruby object containing the data to plot with the properties
that will be passed to the plot command for that dataset. Any Ruby
object can be associated with a DataSet as long as it understands the
to_gplot method.

to_gplot

Within Gnuplot, plot data is read in very simple formats. The
to_gplot method is expected to write the data of the object in a format
that is understandable by Gnuplot. One of the many great things about
Ruby is that methods can be added after the original declaration. The
gnuplot module defines the to_gplot method on the following classes:
Array, String, and Matrix.
Simply define a to_gplot method on your own class to tie the class into
gnuplot.

Examples

Simple sin wave

The following example simply plots the value of sin(x) between the
ranges of -10 and 10. A few points to notice:

The code uses nested blocks to construct the plot. The newly
created object is passed to the block so it can be modified in
place.

Each of the gnuplot plot variables are modified using the
variable name as a method name on the plot object or on the dataset
object. The wrapper also takes care of the single quoting that is
required on some of the variables like title, ylabel, and xlabel.

The plot object simply has an array of DataSets. The
constructor initializes this empty array before yielding to the
block. This method uses the << operator to add the DataSet to
the plot.

When the plot block ends, if an IO object is given to the Plot
constructor, the plot commands will be written to the IO object.
Any object can be passed to the constructor as long as it
understands the << operator.


Gnuplot.open do |gp|
  Gnuplot::Plot.new( gp ) do |plot|
  
    plot.xrange "[-10:10]"
    plot.title  "Sin Wave Example"
    plot.ylabel "x"
    plot.xlabel "sin(x)"
    
    plot.data << Gnuplot::DataSet.new( "sin(x)" ) do |ds|
      ds.with = "lines"
      ds.linewidth = 4
    end
    
  end
  
end

Plotting discrete points

Array data can be plotted quite easily since Arrays have a defined to_gplot method.

Simply pass an array of data to the constructor of the DataSet object or set the data property of the DataSet. In this example, because there are two arrays, each array will be a single column of data to the gnuplot process.

Gnuplot.open do |gp|
  Gnuplot::Plot.new( gp ) do |plot|
  
    plot.title  "Array Plot Example"
    plot.ylabel "x"
    plot.xlabel "x^2"
    
    x = (0..50).collect { |v| v.to_f }
    y = x.collect { |v| v ** 2 }

    plot.data << Gnuplot::DataSet.new( [x, y] ) do |ds|
      ds.with = "linespoints"
      ds.notitle
    end
  end
end
    
Multiple Data Sets

As many data sets as are desired can be attached to a plot. Each of these can have their own plot modifiers. Notice in this example how the data array is explicitly set instead of using the << operator.

Also in this example, the commands are not written to the Gnuplot process but are instead written to a File called gnuplot.dat. This file can later be run directly by a gnuplot process as it contains only the gnuplot commands.

File.open( "gnuplot.dat", "w") do |gp|
  Gnuplot::Plot.new( gp ) do |plot|
  
    plot.xrange "[-10:10]"
    plot.title  "Sin Wave Example"
    plot.ylabel "x"
    plot.xlabel "sin(x)"
    
    x = (0..50).collect { |v| v.to_f }
    y = x.collect { |v| v ** 2 }

    plot.data = [
      Gnuplot::DataSet.new( "sin(x)" ) { |ds|
        ds.with = "lines"
        ds.title = "String function"
    	  ds.linewidth = 4
      },
    
      Gnuplot::DataSet.new( [x, y] ) { |ds|
        ds.with = "linespoints"
        ds.title = "Array data"
      }
    ]

  end
  
end
    

You can also add atribtrary lines to the output 

plot.arbitrary_lines << "set ylabel \"y label" font \"Helvetica,20\""