with a format spec (this should also fix the problem of bar bottoms
not affecting the axis scaling), made the heuristic for determing the
number of axis ticks to aim for smarter, added a new image plugin for
plotting images


git-svn-id: https://flot.googlecode.com/svn/trunk@177 1e0a6537-2640-0410-bfb7-f154510ff394