Honestly, I'm not sure I care about all these weird metrics of soldier performance representing "experience" as if it were an rpg. And I generally love numbers!
XCom already has its own form of experience and improvements, which doesn't involve xp points or level ups.
I think seeing "Cumulative kills over time", and kills relative to the squad. Every time you go on a mission, you log 2 numbers: How many aliens you killed, and what % of all the aliens you killed, average that for every month to get the graph in the current monthly format. Maybe also Cumulative and relative hits could be interesting.
Another thing that would be interesting to me is seeing the progression of stats, where you can see the stats of your soldier at the end of every month. It could be done exactly like the current graph, with each line being a different stat, and the total being the sum of all stats and representing the overall quality of your soldier.