1D Plots

This is the start of a general infrastructure for plots which only distribute their data in one dimension.

The motivation are things like plotting tops and associated descriptions in geophysics files, but also allowing things like axis scatters as used by Tufte.  Potentially can move jitter plots and colorbar plots into this infrastructure.

This is not yet ready to merge.
`x_mapper` and `y_mapper` should also depend on `index_mapper`.