An example vega lite chart with two series that have two different scales. This shows a problem that the scales aren't independent.

example-vega-lite-chart-with-series-that-have-different-scales.vl.json

{
  "$schema": "https://vega.github.io/schema/vega-lite/v2.json",
  "layer": [
    {
      "data": {
        "url": "https://vega.github.io/vega-lite/data/seattle-weather.csv"
      },
      "mark": "line",
      "encoding": {
        "x": {
          "field": "date",
          "type": "temporal"
        },
        "y": {
          "field": "temp_min",
          "type": "quantitative",
          "axis": {
            "orient": "left"
          }
        }
      }
    },
    {
      "data": {
        "url": "data/stocks.csv"
      },
      "transform": [{"filter": "datum.symbol==='GOOG'"}],
      "mark": "line",
      "encoding": {
        "x": {
          "field": "date",
          "type": "temporal"
        },
        "y": {
          "field": "price",
          "type": "quantitative",
          "axis": {
            "orient": "right"
          }
        }
      }
    }
  ]
}