Syntax:
set rgbmax {1.0 | 255}
unset rgbmax
The red/green/blue color components of an rgbimage plot are by default
interpreted as integers in the range [0:255]. set rgbmax 1.0 tells the
program that data values used to generate the color components of a plot
with rgbimage or rgbalpha are floating point values in the range [0:1].
unset rgbmax returns to the default integer range [0:255].