kcms/energy: Remove extra top/right paddings and fix plot area

Previous drawing assumed a centered plot with symmetric x and y paddings, which resulted in extra wasted space on the top and right of the plot.

Rework the plot positioning to allow different paddings on every edge, which will serve to accomodate the axis labels.

This new plot geometry property is also used to better center the placeholder message.

Also limit the min/max graph height so it doesn't get too big on maximized pages, or too small on compact ones.

On top of the refactor set at !226 (merged)

Bugs fixed

BUG: 443569
FIXED-IN: 6.4

Screenshots

	BEFORE	AFTER
small size
max size