Graphline, extract data from a plot

A little while ago I needed to extract data from a plot but no tool I could find actually worked on awkward data, quite a common problem.

So I made my own method, which has subsequently proven useful.

It seems reasonable to do a quick manual and make the whole thing available, hopefully helping others.

Above is from result of the demonstration, a pretty good overlay.

Manual is here as a PDF

Software, demo and docs is here as a ZIP (252k, with images)