Line Chart with Varying Size (using the trail mark)

{
  "$schema": "https://vega.github.io/schema/vega-lite/v2.json",
  "description": "Stock prices of 5 Tech Companies over Time.",
  "data": {
    "url": "data/stocks.csv"
  },
  "mark": "trail",
  "encoding": {
    "x": {
      "field": "date",
      "type": "temporal",
      "axis": {
        "format": "%Y"
      }
    },
    "y": {
      "field": "price",
      "type": "quantitative"
    },
    "size": {
      "field": "price",
      "type": "quantitative"
    },
    "color": {
      "field": "symbol",
      "type": "nominal"
    }
  }
}