Continuous linewidth scale — maps data values to a stroke-width range
(R’s scale_linewidth). Unlike the position (x/y) scales, with_limits
squishes out-of-range values to the nearest limit rather than filtering
them. with_transform is applied to the value and the limits before
rescaling into range, inside map /
map_to_size — it does not override
Scale::transform (that’s for pre-stat domain transforms like
scale_x_log10, which linewidth has no use for).