Interval x_extent = 0.5 * linethickness *Interval (-1,1);
Interval y_extent = Interval (pmin, pmax);
Box line_box (x_extent, y_extent);
Interval x_extent = 0.5 * linethickness *Interval (-1,1);
Interval y_extent = Interval (pmin, pmax);
Box line_box (x_extent, y_extent);