[ferret_users] invert color key

[Jonathan Winter <jwinter@xxxxxxx>]

I'm trying to make some proper energy plots with consistent sign
conventions.  Basically, I'd like a color bar with -170 at the top, -20
at the bottom, intervals of 10.

I've reversed the default palette to get -170 red and -20 purple, which
works, but I can't find a way to get the color bar and labels to
reverse.  I've tried every possible combination of levels, this is the
one that I thought would work:

fill/level=(-20,-170,-10)/palette=inverse_rnb/nolabel 
var_obs[i=6:83,j=6:53]/21600, x_page, y_page

but unfortunately doesn't.  thanks in advance for the help.

jonathan