Currently at a merge point the in graph, you only see the modified file list for the commit when compared with one of the parents. Some other git GUIs automatically show the changes between both parents (e.g. GitExtensions) or make it easy to show them (e.g. gitg)

It would be very useful if Git Graph supported this too. An example from GitExtensions is in this image:

The current workaround is to use Ctrl-Click to select the alternate parent, but that can require scrolling a long way down the tree if the commit being merged is far back in history.