I installed gnuplot gem, and I am getting some syntax errors when I try to 
run the samples on the projects web page.

>ruby test.rb
c:/programs/ruby/lib/ruby/gems/1.8/gems/gnuplot-2.1/lib/gnuplot.rb:20:in 
`popen': No such file or directory - which gnuplot (Errno::ENOENT)
 from 
c:/programs/ruby/lib/ruby/gems/1.8/gems/gnuplot-2.1/lib/gnuplot.rb:20:in 
`open'
 from test.rb:2
>Exit code: 1

my test code is copied directly from the gnuplot ruby project web page 
except the added require 'gnuplot' which is not mentioned in the example. I 
think I am missing something simple here.

require 'gnuplot'
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

any help is appreciated.