When reading electronics datasheets, I often have to read values off charts that have sparse gridlines. I can save a screenshot of the chart using Greenshot, then input the image to WebPlotDigitizer, click on the corners of the graph, say whether they're linear or logarithmic axes, and click a point on the graph to get the actual value of the point along that line (or digitize the entire graph so I can use it in a spreadsheet).
I've also used things like Meazure, an overlay screen ruler, when I just want to read one point. It lets you drag around points, showing a zoomed-in pixel view for accuracy, and measures the distance between them in various units. For graphs I have to manually calculate the proportions to get the actual value, though.
Is there anything that combines these concepts, with a screen ruler overlay that I can click the 4 corners of a chart and then just read points off it in the graph's units, without having to take screenshots, open files, etc?