I’m guessing you are talking about something similar to Gimp or Photoshop?
You you can set the min and max for the current view using the view tool (ctrl-F1). However, to display a full histogram would require a full load of the image (as opposed to just loading the required slice via memory mapping). We would not want to do this by default since we would loose the snappiness of mrview. However, I guess we could do a full load on the image if a histogram tool or button was pressed. I agree that it might be a nice feature to have, however given our limited spare time we have for developing new features, I can think of quite a few more features/tools/commands that would be more heavily used by the average user. Coding up a image histogram in a pop-up (either voxel or fixel-based) would not be too much work, however having a slider to then set the min/max of the image would be a little more involved.
As a side note, you can use
mrstats image.mif -histogram hist.txt to generate a histogram (as a text file) from any voxel of fixel image. This can also work in conjunction with an input mask.