Plotting Pixel Intensity

[Doev]

Hello - How does one obtain and plot pixel intensity of a selected area on the screen?
Ideally, the script will prompt the user to select a small area (<10 pixels or so) by means of a cross bar, then the average pixel intensity of that selected area will be obtained and eventually plotted at fixed intervals (say every half second). Assume images/video is gray scale (8-bit).

I would greatly appreciate any assistance.

Thanks.