Digitizing a line plot from an image can be done in MATLAB using functions from the Image Processing Toolbox. You can read the following in-depth blog posts that demonstrate how to extract data from a line plot.
As noted in the blog post, "automatability[...]depends markedly on the ability to exploit similarities in the data set (images)." This is to say that the ability to automate depends on the amount of preprocessing that must be done to make each image resemble each other. For example, if one image has a large watermark across its entire graph, this could entail a different set of preprocessing steps as compared to an image without such a watermark.
However, there are also many community contributions at MATLAB File Exchange that offer solutions for this feature:
