Overlays: points on top of lines, line on top of areas

[kanitw]

Line chart often have points marks on top by default. Maybe add a config to automatically layout point on top of line to make it hoverable?

More importantly, when there is only a single data point. No line would be draw. Augmenting a point is very useful in this case.

{
  "description": "Line chart of Horsepower over time",
  "data": {"url": "data/cars.json"},
  "mark": "line",
  "encoding": {
    "x": {"field": "Year", "type": "temporal", "timeUnit": "month"},
    "y": {"field": "Horsepower", "type": "quantitative", "aggregate": "mean"}
  }
}

If you change the mark to point, you can still see the point.

- [ ] allow **overlaying point on top of line/mark** so that we can see the data if there is only one point and also make it easier to hover line data. - [ ] add `config.mark.overlayPoint` - [ ] support shape with lines / areas (which modifies `point`) - [ ] apply the overlayed shape to legends - [ ] similarly for area, one might want to overlay both point and line on top of area

<img width="319" alt="vega_editor" src="https://cloud.githubusercontent.com/assets/111269/12525909/2c0fcb1a-c11d-11e5-8c50-a62aca237283.png”/>