Sublime Merge can show the diff between any two commits and it can show the diff between uncommitted changes and the most recent commit, but it cannot show the diff between uncommitted changes and older commits. I hope you will add that feature.
It can be very helpful in following history and trying to undo a few changes while keeping other changes.
From a UI perspective it means allowing allowing the user to select the top item in the graph (e.g. “14 unstaged files”) and shift-click to select any older commit. So it adds uniformity to the UI and no clutter.