Allow visual "options" at each step in the pipeline?

[jhofman]

At some point it might be nice to let people control various visual options at each point in the analysis pipeline, for instance:

• Rescale an axis to zoom to what's left, e.g. if a facet is dropped from filtering (or not)
• Showing error bars on summarizes (or not)

Probably a good to go with "what would happen if you ggplot-ed only this frame" as a default? (Sometimes vegalite and ggplot2 will differ in this and then life gets hard.)