fixed arg validation in grob.transformmode
made Points copyable
added x/y properties to Bezier

- allows the new plot() command to override location when e.g., drawing
multiple copies of a shape primitive
plot() can now reposition grobs

- plot() now accepts a pair of x/y coordinates as the second and third
positional args
- dropped the `inherit` arg and collapsed the `live` option into kwargs
- added the inexplicably missing docstring