How to bind an image angle in a widget for HUD


I want to display a barometer (as kind of an energy source) in my HUD.
Problem is: I dont know how to rotate an image in a widget. There is a way to change the angle in the *Render Transform" panel, but its a pity you cant bind that.

While I could use a Progress Bar to bind it directly there seems to be no option to bind the angle to a variable.

How would I do that then?