Class PercentScreenPlotter

  object --+    
           |    
_BasePlotter --+
               |
              PercentScreenPlotter

A class to plot multiple series of Calculator data as %Actives Found
vs %Screen.

API example
-----------
Where <enrich_calc1> and <enrich_calc2> are instances of Calculator:

enrich_plotter = enrichment.PercentScreenPlotter([enrich_calc1, enrich_calc2])
enrich_plotter.plot() # Launch interactive plot window.
enrich_plotter.savePlot('my_plot.png') # Save plot to file.

There are six line styles defined by default.  Plotting more than
six results cycles through the styles.