To digitize the whole curve you have to convert all data pixels to background pixels. Of course, this is quickly done by setting a large Width value, but you have to avoid to pick wrong points such as black labels or arrows. During the scan you can change the Width and continue with small values in regions close to parts of the picture which are no wanted data points.

After a while I managed to scan all pixels of the bottom curve of the picture so that the curve has disappeared completely (all pixels converted to background):

If you have done something wrong (e.g. deleted too many points) you have to reload the bitmap and click on the Clear button to delete the list of digitized pixels. If you load exactly the same bitmap you do not have to do the calibration again.