Sometimes you just don't have the raw data and all you have is a plot. This tool (link below) does a reasonable job on a variety of plots.
I tried it on this chart exported from Mathematica as a JPEG.
You have to calibrate your Axes and the pick the trace color of the plot which it offers in a menu of dominant colors derived from the chart. It's pretty cool that the points I got back had a correlation of nearly 1 for the original data in Mathematica.
The tool also has an option WebCam option and can import and export JSON which is nice as well. Overall I give this an overall rating of 9.5/10! I still haven't found my holy grail screen scraper yet but this one works well for manual scraping of charts.
Also see this article: http://connectedresearchers.com/extracting-data-from-plots-images-and-maps-with-webplotdigitizer/ for more information about this tool.
Author: Ankit Rohatgi
Date: May, 2016
Location: Austin, Texas, USA