DataThief is a nifty Shareware utility for extracting the coordinates from an image of a graph. After a plane is defined (by marking three coordinates on the picture), one can mouse over the gra开发者_如何学Cph to find the numerical coordinates of each points.

Can it be automated? Online documentation seems to be nil.

Follow the color method

First, open up the picture in DataThief.

I found it helpful to zoom in (Action Menu).

Next, I set the coordinates of some reference points on the graph, by moving the colored crosshairs.

The vertical red/green rectangle is a progress bar relating how fit, squared, orthogonal the reference points are.

I placed the Start, Color, and End markers, choose Trace (Action Menu), and then Export (File Menu).

The result was a tidy text file with my data points. Great! Now to capture some useful data.

Scatter plot method

Tracing a continuous graph appears to be just one (the upper right button) of DataThief's four data acquisition modes. Let's match the data points via the scatter plot option (lower right button). Drag the crosshairs from the gray box over each data point.

I am interested in automating this process for a graph with hundreds of data points?

I clicked on the Settings tab, fiddled with some options, and got the next graph via the Show button.

Good, the data are points can be isolated via color recognition.

The question

How do I automate the selection of an arbitrary number of points?