non-color-scaled rgbimage plots

[Dima K. <gn...@di...>]

Hi. These days a common plotting use case for me is to draw an image
from a file, with some data plotted on top of it. I do something like
this:

  plot "xxx.png" binary filetype=auto flipy with rgbimage, "yyy" with points palette, ...

This works OK. There's an annoyance however: the points in "yyy" have a
3rd column, rendered as a color, and I want gnuplot to autoscale these
colors. But the pixels in "xxx.png" also have colors and gnuplot
interprets these colors as data too, and these 0-255 color values are
included in the autoscaling along with the 3rd column in "yyy". So for
instance, if the 3rd column in "yyy" is in [0-1], then the image is
plotted normally, but the "yyy" points all look black. Conversely, if
the points in "yyy" are in [10000-20000], then the points all look
yellow and the image is all black.

Is there a way to instruct gnuplot to just plot the image as is, without
trying to interpret the pixel colors as data? If not, can we add such an
option? The current behavior is definitely good for some use cases, but
not for all of them.