One technique in estimating areas from images is through the use of Green’s Theorem. Given a closed region R on the xy plane, its area can be calculated as
Scilab is a programming platform that has similar functions with that of MATLAB, only that it’s open-source.
Images, in computer ‘language’, are just matrices of numbers corresponding to pixel values. Scilab is a useful tool in manipulating images because it can do a lot of matrix operations with only a few lines of codes.
This activity is from one of my applied physics courses which deals with image processing methods.
Given digitally scanned plots such as hand-drawn historical or old plots or even the computer generated ones, you could get the values by simple ratio and proportion. This sounds so simple but takes a lot of time and focus.