I suspect the problem is that matplotlib needs constant-length arrays to plot two on the same plot. For each 'tick' of the animate function it will need to know what value to write for that instant.

Subplot is designed to help you focus on creative writing. English US Edition. ... Subplot helps writers focus on characters, plot. By Brendan Wilhide. NOV 21, 2011 8:30 pm PST. At a glance.