Add Modding API for GUI Plots like Electric

[redrook]

Allow mods access to plot widget to create custom GUIs for stats and monitoring like electric grid monitoring or "P" production panel.

Why: Some mods make an extensive use of steam as power for a long time in the game, with plot widget mods could add a monitor analogous to electric to track steam production & consumption A) per building type, not just in total, and B) per connected component of pipes and pass-through buildings, not just for the whole surface.