Margin behavior

Readyplot now handles margins in bar, box, and strip plots much better. The distance between the first bar and the dependent variable axis is now the same as the distance between groups of bars, similar to how other graphing software would do it. Also changed where lines and patches are drawn (after justplot() which now calls super().justplot() to manage patches and extra lines consistently), in addition to some better zorder handling for subplots, which also needed a lot of work for margins to register correctly.