fixing cached image invalidation in colormapped image plots
fixing cached image invalidation in colormapped image plots

ContinuumIO/chaco,burnpanck/chaco,tommy-u/chaco,ContinuumIO/chaco,ContinuumIO/chaco,burnpanck/chaco,tommy-u/chaco,ContinuumIO/chaco,burnpanck/chaco,tommy-u/chaco