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.

Advanced Matplotlib Concepts In this section we cover some more advanced topics which you won't usually use as often.

To plot multiple plots on the same canvas use the subplot method. This takes arguments: nrows, ncols, plot_number, where the plots are placed in a 2-D array on the screen.