Set the Shape to a Circle, make it a dot of some sort
In the Paint tab, select the "Color" option then press the "Calculator" icon on top, this will turn the paint into a formula
Use this formula $if(bi(charging) = 0, #FF00FF00, #00000000)$ then replace "FF00FF00" with the color your want, this will make the shape transparent when not charging and use your color when charging