Support bandPosition for area marks

[djbarnwal]

Enhancement Description

bandPosition is supported for bars and line but not for area marks. It would be great to support area as well. Area charts are interchangeably used with line charts so there's a wide use case here as well

{
  "$schema": "https://vega.github.io/schema/vega-lite/v5.json",
  "description": "Google's stock price over time.",
  "data": {"url": "data/stocks.csv"},
  "width": 900,
  "transform": [
    {"filter": "datum.symbol==='GOOG'"},
    {"filter": {"field": "date", "timeUnit": "year", "range": [2006, 2007]}}
  ],
  "mark": "area",
  "encoding": {
    "x": {"field": "date", "type": "temporal", "timeUnit": "yearmonth", "bandPosition": 0.5},
    "y": {"field": "price", "type": "quantitative"}
  }
}

Replacing area with line in the above spec works out fine.

Checklist

• I checked for duplicate issues.