[feature request] Provide set origin and anchor capabilities to `plot`

[tapyu]

AFAIK we cannot set an origin to plot.plot. It is automatically set to (0,0), let alone set an anchor with respect to that origin. Although we can workaround it by translate the coordinate system within a group, it would be interesting to be able to set the origin as we do with content, circle, ...

Moreover, in addition to the anchors we can set via add-anchor, having built-in named compass anchors ("west", "north", etc.) is very effective when putting stuffs around the plot axis.