Change changeset being compared.
XCode has a nice feature during comparing files.
It lets you change the one on the right side (so the one from version control with a slider),
The effect is that you can easily find the spot in time where something has changed. Such feature would need the positions in both files to remain stable (so not jump to the top).
Piotr Mis commented
Maybe just set of icons for left panel:
Previous/next revision globaly - it will just switch left panel to previous/next revision of file (whatever branch)
Previous/next revision in branch of currently displayed revision
Previous/next revision in branch of currently displayed revision in right panel file (this will work also for workspace comparison - in such case shall it will result in current workspace branch)
Additionally I would add icon: "Select revision" after clicking which dialog window similar to the one displayed after choosing "Diff revision..." option from context menu in "Items" view.
This dialog could be two areas (like "Diff revision..." in which currently both revisions to compare can be chosen) or only one (if you plan to allow changing only left panel). This you could consider even if you do not add Prev/Next. It will save a lot of clicking what trying to compare historical revisions - now we always must go back to "History" or "Items" tab.
I hope I described it in undarstandable way. If not please let me know I will try to be more clear.
I guess you mean the one on the left, since the one on the right is typically the working copy, correct?
A slider is a good option for a linear history, but not valid for branches (not very comfortable) which is what you tend to use with Plastic.
The idea is good, but we have to find a way to make it happen.
Any additional ideas would be great.