So far as I can see, the widget which draws the end screen:
https://github.com/ZeroK-RTS/Zero-K/blob/33f993f9a25cbdaf5624a1309a8ff1ed7639b540/LuaUI/Widgets/gui_chili_endgraph.luagets its statistics from a call to Spring.GetTeamStatsHistory, which can be found here:
https://github.com/spring/spring/blob/2d04655a59405bb5f54eb3d6795cf754763c45ea/rts/Lua/LuaSyncedRead.cppThe latter does not report quantity of metal reclaimed, so this change is not trivial to make so far as I can see. Might well be possible, but you would probably have to either make a change Spring-side or write a hack ZK-side.
EDIT: Looking at this definition
https://github.com/spring/spring/blob/036b798fefe0e4a3bc82be8ecb8aa8806a6a9263/rts/Sim/Misc/TeamStatistics.cppit appears that Spring fundamentally doesn't record quantity of metal reclaimed, at least not in this way. I wonder how the reclaim award actually works, might be a clue.
EDIT 2: Wow OK. If I'm reading this right:
https://github.com/ZeroK-RTS/Zero-K/blob/33f993f9a25cbdaf5624a1309a8ff1ed7639b540/LuaRules/Gadgets/awards.luathe reclaim award works by checking every feature (wreck, rock, tree, whatever) in the game and asking it how much metal was reclaimed from it. That doesn't seem useful for graphing.
I fully agree that the graphs at present could use improving and a metal reclaimed graph would be neat, but it might not be easy to accomplish.