如何在vega-lite中添加具有相同比例的2个系列的辅助Y轴？

{
  "data": {
    "values": [
      {"x": 1, "y1": 10, "y2": 20},
      {"x": 2, "y1": 15, "y2": 25},
      {"x": 3, "y1": 20, "y2": 30},
      {"x": 4, "y1": 25, "y2": 35},
      {"x": 5, "y1": 30, "y2": 40}
    ]
  },
  "mark": "line",
  "encoding": {
    "x": {"field": "x", "type": "quantitative"},
    "y": {"field": "y1", "type": "quantitative"},
    "y2": {"field": "y2", "type": "quantitative"}
  }
}

{
  "layer": [
    {
      "mark": "line",
      "encoding": {
        "y": {"field": "y1", "type": "quantitative"}
      }
    },
    {
      "mark": "line",
      "encoding": {
        "y": {"field": "y2", "type": "quantitative"},
        "y2": {"field": "y1", "type": "quantitative"}
      }
    }
  ]
}

const vegaEmbed = require('vega-embed');

const spec = {
  // Vega-Lite 规范文件
};

vegaEmbed('#chart', spec);