I digitised the following image using plot digitiser, which is a java program for getting numeric data from images. The horizontal image is scaled from 0-200 and is around 1272 pixels in length, this was acquired using print screen from pdf version of paper. I viewed the image on a flat–screen computer monitor (HPCompaq LA2505wg) with no additional magnification. The positions of the ordinate values are known so can be used to assess the accuracy of digitising as shown in the second image.
So in this case maximum error in measurement was around 0.5% of fullscale. Error across the measurements looks to be systematic so might have more to do with positioning of calibration markers than finding and clicking centers of the data points.