yo can do this in photoshop or gimp. I tend to use PSP for something like this, just because I find the prog quick to load. I've never used gimp, but it's free. PSP used to have full free versions available, but that may have changed since Corel took Jasc over and they may now be time limited.
You may still be able to find full free PSP7 which at one time was even more powerful than Photoshop for its features.
The main problem could be the background graph ie the graph scale and axis etc. if both graphs have the same background then it should be relatively easy. So 2 similar hlog files shouldn't take long.
In psp you'd open both images, CTRL C (copy) one of the images and then CTRL L to paste as layer on top of the other.
In Photoshop you can just drag one image on top of the other.
Then go to layer properties -> opacity. Drag the slider to reduce the transparency until the bottom layer image becomes visible.