Check this other question. I did not try those methods, but I hope they help you!
As a workaround, and if there are only one plot line per graph, you could try to use imread to load the png files and then delete all the pixels which are not part of your data, but I would not recommend it.
If they are saved from a matlab script that you still have, change it to save the graphs as .fig. THen you can directly load and combine them without problems using "hold on"
