Fixed 'Image Plot' example in the tutorial docs.
Fixed 'Image Plot' example in the tutorial docs.

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