Render custom graphic over sidebar/game view

[yuri624]

I'm trying to render a custom graphic. Let's say I have that pink box. How can I render it on top of the sidebar next to the credits (1), over the build icons (3) and maybe the game view (2).
All I found was code in power.cpp, radar.cpp and sidebar.cpp that mostly doesn't seem to do anything. I can comment out a lot of it without seeing any change ingame.

Any idea how to do that?