[matplotlib-devel] Matplotlib plot problem

I'm sure this is a simple problem, but I've been going around in circles several days trying to figure this out.  
Here's my code.http://pastebin.com/n83dGhG4

Basically, I'm trying to get my pyqt4 button signal to execute the plot command (line 122).  A lot of this code was copied from the matplotlib example page.  If I uncomment line 148, the figure will successfully plot.  I don't understand why what I'm doing on line 122 isn't equivalent to line 148.
thx for your help.Matt