Example vega-lite chart with two time series and multiple layers.

example-vega-lite-chart.json

{
  "$schema": "https://vega.github.io/schema/vega-lite/v2.json",
  "data": {
    "url": "https://vega.github.io/vega-lite/data/seattle-weather.csv"
  },
  "layer": [
    {
      "mark": "line",
      "encoding": {
        "x": {
          "timeUnit": "month",
          "field": "date",
          "type": "temporal"
        },
        "y": {
          "aggregate": "mean",
          "field": "temp_min",
          "type": "quantitative"
        }
      }
    },
    {
      "mark": "line",
      "encoding": {
        "x": {
          "timeUnit": "month",
          "field": "date",
          "type": "temporal"
        },
        "y": {
          "aggregate": "mean",
          "field": "temp_max",
          "type": "quantitative"
        }
      }
    }
  ]}