Runs entirely in your browser (nothing is uploaded to server). You can download this html and run it on your own machine if you like.

Instructions: Choose an image or PDF, set the X axis type (linear or log), then click Calibrate X. Click on the image to set the two points for calibration (X1 and X2). Repeat for the Y axis. Note: the axis type must be set before calibrating that axis — the dropdown will lock once calibration is complete.

After calibration, clicking the image adds red digitized points. Use the zoom window to fine position.