def plot opts={}, &block
          index_as_default_x_axis(opts) unless x_axis_defined?(opts)

          if opts[:categorized]
            plot_with_category(opts, &block)
          else
            plot_without_category(opts, &block)
          end
        end
Plots a DataFrame with Nyaplot on IRuby using the given options. Yields
the corresponding Nyaplot::Plot object and the Nyaplot::Diagram object
to the block, if it is specified. See the nyaplot docs for info on how to
further use these objects.

Detailed instructions on use of the plotting API can be found in the
notebooks whose links you can find in the README.

== Options

* +:type+  - Type of plot. Can be :scatter, :bar, :histogram, :line or :box.
* +:x+ - Vector to be used for X co-ordinates.
* +:y+ - Vector to be used for Y co-ordinates.

== Usage
# Simple bar chart
df = Daru::DataFrame.new({a:['A', 'B', 'C', 'D', 'E'], b:[10,20,30,40,50]})
df.plot type: :bar, x: :a, y: :b