Finish off a javascript app

I have started to write a javascript app to count the number tree rings there are on a tree. I want to do this by the user being able to highlight a horizontal line that marks the begining and end of the section that is to be counted. I want the count to done by looking at the change in colour along the line. So I only want significant changes in colour/shade to be counted. Like I have said I have already written quite a lot of the code, I just need someone to finish it off.