implement logarithmic color_continuous_scale

[eisenlohr]

It would be helpful to have an option (named, for instance, log_color) that allows using color maps on logarithmic scales, similar to log_x, log_y, log_z. This would essentially rescale the standard linear color range to the corresponding log values, i.e. coloring of v would be linear in log(v).

[cemde]

is this related to #3735 ?

[eisenlohr]

is this related to #3735 ?

Yes, seems to be the same issue. What is missing is a simple way to have the color scale be equidistant on a log (in contrast to linear) scale.

Let's say the color range is between values A and B.
Linear example: value (A+B)/2 is colored using the midpoint of the color scale.
Log example: value sqrt(A*B) is colored using the midpoint of the color scale