[matplotlib-devel] plot argument handling

In svn 7298 I changed a key chunk of code that was working fine, so I 
would like to explain why I fixed it when it wasn't broken, and ask for 
review and testing, or at least keeping your eyes open for anything I 
may have overlooked.

In the course of making the very simple change to support scalar 
arguments to plot (for plotting a single symbol), I found, as I have 
many times before, that the argument handling was rather complex, 
repetitive, and hard to follow.  I couldn't resist seeing if I could 
simplify it, in the hope that this would make it easier to maintain. 
The result is a reduction in LOC and a codepath that to my eye, at 
least, is easier to follow.

Eric