Total number of pixels
Thanks for the reply. I am familiar with the CiC tutorials, but, as they say, I think it's a bit more complicated than that.
*** nerd alert ***
Another way of asking is "What is the vertical scale?"
The total number of pixels is fixed for any given image, so the Area Under the Curve (auc) is also fixed. However, if I look at different histograms of the same resolution, that is obviously not the case. I can see broad histograms with the auc filling half the space and spikey ones with a much smaller apparent auc.
Similarly, two images of the same or similar scenes show much the same histogram, even if they are taken at very different resolutions. That makes sense - there would not be much sense in having a low res histogram crawling along the bottom of the graph.
So, I it could just scale so that the most intense point of the most intense channel hits the top of the graph. Except it doesn't, because it's easy to find histograms which clearly continue over the top of the visible graph.
It's not really important from a photography point of view, it just bugs me that I don't know.
Incidentally, and I expect this has been pointed out before, the histograms for the same image in LR4 and ACR 7.4 (as in PSE 11) are different.