Solved
Plot legends in Prime
You cannot put legends on Plots in Prime!
But if you can't, but really want to, then you can!
Quite a nice idea, even though it would look a bit strange if you use real units in your plot (which you should), too.
P.S.: Missing a secondary y-axis! The chart component is not an option as it does not scale properly according to screen resolution (a problem with small 4K displays) and does not support units.