#138 double slider - color range lopsided

[Rob Lintern]

double slider - color range lopsided (ex. bar is mostly
white with a tiny bit of blue at one end)
- solution 1 - use some type of "log" scale for some
attributes (problem: how to automatically determine if
an attribute has this "log" distribution)
- solution 2 - evenly distribute the colour across entire
range of values (doesn't seem to do this now)