For the purposes of avoiding another full format call we can stash the
last rendered 1d xy pre-graphics formats as
`IncrementalFormatter.x/y_1d: np.ndarray`s and allow readers in the viz
and render machinery to use this data easily for things like "only
drawing the last uppx's worth of data as a line". Also add
a `.flat_index_ratio: float` which can be used similarly as a scalar
applied to indexes into the src array but instead when indexing
(flattened) 1d xy formatted outputs. Finally, this drops the way
overdone/noisy `.__repr__()` meth we had XD
b762cf0456