http://comments.gmane.org/gmane.comp.gis.gmt.user/20760
makecpt -Z -Chaxby -T-75.0/0.0/25.0 -N > paletteTwice.cpt
makecpt -Z -Crainbow -T0.0/150.0/50.0 >> paletteTwice.cpt
psscale
-Cpalette.cpt -D3.2/0.4/6.0/0.2/h -L -V
> non_symmetric_colorbar.ps
makecpt -Z -Chaxby -T-75/0/25 -N > cpt1
makecpt -Z -Crainbow -T0/150/50 -N > cpt2
cat cpt1 cpt2
> cpt
