When multiple lines are being shown within a single axes, it can be useful to create a plot legend that labels each line type. Again, matplotlib has a built-in way of quickly creating such a legend.

A python class to facilitate annotating multiple lines using matplotlib. - GitHub ... # Plot as many lines as needed new_graph.plot_line(x1, y1) new_graph.plot_line(x2, y2) # Open window and display ...

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.