[Rd] RFC: log='z' for image, contour, persp?

[Duncan Murdoch]

I've been thinking of adding the possibility of including "z" among the 
axes to be logged in image, contour, and persp.  In the first two, it 
would only affect where the breaks were set if they are calculated 
automatically; it would have a bigger effect in persp.

For example,

image(x, y, z, log="z")

would set 12 colours evenly spaced on a log scale of the z values.  (12 
because that's the default).

We already support

image(x, y, z, log="x")

to scale the x axis (though there's a spurious warning; I'll fix that).

image(z, log="x")

fails because it tries to take a log of zero.

Does it seem like a good idea for these 3D functions to support log="z" 
the way 2D functions do?

Duncan Murdoch