Actually, Rawtherapee has this feature. By default, the Rawtherapee histogram shows the after processing result, as seen top right of the editor mode by default here is the full gamma processed result, top picture.
To see the linear unprocessed raw histogram, just click the 2x2 bayer array icon to the right of the histogram for the raw histogram display, which is true uprocessed raw data (bottom picture, featuring yellow arrows pointing to it.
Processed Histogram:
Raw Histogram:
