Going to assume that the set of line children are the same for all lines.

For now I will use the same margin above/below lines regardless of where they sit on the page (first, last or other).

'Rectangle' is a sealed class. What the fuck is wrong with letting someone derive from it? Fucking arseholes...

Got a panel to hold/measure/arrange all drawing panels/rectangles for a single line and a panel to build/measure/arrange collection of these panels. Placement and painting basics sorted.