stat_summary_bin no default summary function

[keatingw]

First off, thanks so much for this project - an implementation of ggplot2 in python is absolutely amazing.

I noticed that stat_summary_bin does not have a summary function set by default, raising a PlotnineError 'No summary function'. This seems to run contrary to its docstring stating that "mean_cl_boot" is the default. When using stat_summary the stated behaviour is used.

Having a quick glance at the source code it seems like this is just the matter of setting the default 'fun_data' to None in line 92 of stat_summary_bin.py.

Not a problem if this behaviour is intentional to force the user selection, though it may be best to adjust the docstring in this case.

Cheers!