plot.remove() causes scale to resize

Before, calling `plot.remove()` would cause the data to disappear, but the scale would not resize. This branch fixes that issue by having `Plot` signal all of its projectors that it's gone on `remove()`, and reverses the operation on `_anchor()`. 

Close #554.
since `_getExtent` is potentially an expensive call, maybe we should have it check if its anchored before bothering to compute extent? and then just return early?