FITS histogram limited scroll and zoom

It is usually only sensible to scroll (pan) and zoom over the histogram data where there is data. The frequency is never negative and the intensity is rarely negative (if at all). To limit the user to the existing range will help to keep a clear focus. This is possible using tips in https://www.qcustomplot.com/index.php/support/forum/486 and I'm preping a MR "FITS histogram limited scroll and zoom" for it.

The coming MR assumes that there is no negative pixel data in the image. Please provide me with an example that has and we can patch for it.