As explained earlier, a merged control curve is attached to the image after exiting the histogram main dialog window with a click on [OK]. This control curve may, of course, have little in common with the original control curve. To be able to edit this new control curve afterwards with any control curve editor, without danger of colliding with the histogram settings, all you need to do is click on this icon: The original control curve will be attached to the image and can be edited.
After the modification, the control curves should either be merged (see above) or the histogram editor should be opened in order to adapt the histogram settings to the newly created histogram.
