Hot key to toggle log scale is broken

[Ethan A M. <merritt@u.washington.edu>]

Since I never use the logscale toggle keys ('l' and 'L') I hadn't realized
they are broken. The problem is that coloring options have proliferated, and
many kinds of plots now use the palette for things other than Z-coordinate.

For example, consider the last plot in heatmaps.dem.
One might plausibly want to toggle log scaling on the colorbar but you can't,
because the hotkey tries to toggle Z also (or instead?)
Many other examples of confusion are possible, also in 2D now that
palette coloring can be used in 2D also.

I propose to restrict the effect of 'l' and 'L' to the X,Y axes in 2D
and the Z axis in 3D.  We can add another hotkey, probably 'c', to 
explicitly toggle log scaling on the colorbar in both 2D and 3D plots.
Yes, this is a change.

Is that OK with everyone?

-- 
Ethan A Merritt
Biomolecular Structure Center
University of Washington, Seattle 98195-7742