I'm writing a program where the user can create annotations while interactively zooming and panning the graph. I kept getting errors when my panning would for the arrows to point toward the right. you can recreate the error by going to the example 'annotation_demo.py' and try to make the arrow in Figure 1 point toward the right.
I looked in the source files and I think I've found the cause. A line in file patches.py, class YAArrow, function get_verts(self) needs to be changed from
r = math.sqrt((y2-y1)**2. + (x2-x1)*2.)
r = math.sqrt((y2-y1)**2. + (x2-x1)**2.)
This change seemed to fix the errors I was getting.