The cbrange direction can not be controlled by swapping the minimum and maximum values in gnuplot 5.1 with Windows.
It seems that when creating the colorbox (vertical on the right side of the plot) the higher value is always on top and the lower value is in the bottom.
Compare this with xrange where the axis direction will be changed when swapping the minimum and maximum values in "set xrange [xmin:xmax]"
If you change the direction of the cbaxis range, what are you expecting to happen to the plot itself? Would the colors change? Or are you just expecting the colorbox to be drawn upside down? Both?
How would this interact with "set palette negative"?
New command option
set colorbox {no}invert