Ruby Gnuplot – How To

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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. Now rdp maintains it. See also the changelog for more detail.

Pre-requisites and Installation

1. Install XQuartz from here:

http://xquartz.macosforge.org/landing/

2. Install gnuplot with homebrew:
brew install gnuplot --with-x11

3. Install gnuplot gem:
gem install gnuplot

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 separate 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:

1. Instantiate a Plot or Splot object and set parameters by gnuplot variable name.

2. 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.

3. 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. DataSet s 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 @DataSet@s. 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.xlabel "x"
    plot.ylabel "sin(x)"
    
    plot.data << Gnuplot::DataSet.new( "sin(x)" ) do |ds|
      ds.with = "lines"
      ds.linewidth = 4
    end
    
  end
  
end

Or you can write it out to a file (the above snippet displays the graph, in Linux, but in windows you’d need to write it to a file).

See the file examples/output_image_file.rb.

Plotting discrete points

Array data can be plotted quite easily since @Array@s 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.xlabel "x"
    plot.ylabel "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 arbitrary lines to the output

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

See more in the examples folder. Also since this is basically just a wrapper for gnuplot itself, you should be able to do anything that it can do (demos:
http://gnuplot.sourceforge.net/demo_4.4/ )