De-hack legend creation

The information to create the legend is currently attached to one line, and the function `legend()` is called without arguments. This is somewhat hacky and the results are not perfect. There is one additional data-point on the line.

Maybe create invisible proxy object. Recipe:
http://matplotlib.sourceforge.net/users/legend_guide.html

Problem: The following implementation is impossible:
Therefore collect all information for the legend in `action_graph(...)` and call `legend(...)` with all necessary arguments there.