Plot timeseries of NO2 for Susanna Berkouwer from GEE

no2.js

var COLOR = {
  xinxiang: 'ff0000',
  milan:    '0000ff',
  tehran:   '00ff00'
};

var xinxiang = ee.Feature(    // xinxiang
    ee.Geometry.Rectangle(113.6, 35.1, 114.4, 35.5),
    {label: 'xinxiang'});
var milan = ee.Feature(  // milan
    ee.Geometry.Rectangle(8.9, 45, 10, 45.4),
    {label: 'milan'});
var tehran = ee.Feature(  // tehran
    ee.Geometry.Rectangle(51.2, 35.6, 51.5, 35.7),
    {label: 'tehran'});
var myRegions = new ee.FeatureCollection([xinxiang, milan, tehran]);

// Sentinel-5P NRTI NO2: Near Real-Time Nitrogen Dioxide
var collection = ee.ImageCollection('COPERNICUS/S5P/NRTI/L3_NO2');
var sno = collection.filterBounds(myRegions)
    .filterDate('2018-09-01', '2019-03-01')
    .select('NO2_column_number_density');

var densitySeries = ui.Chart.image.seriesByRegion({
  imageCollection: sno,
  regions: myRegions,
  reducer: ee.Reducer.mean(),
  band: 'NO2_column_number_density',
  scale: 200,
  xProperty: 'system:time_start',
  seriesProperty: 'label'
});
densitySeries.setChartType('ScatterChart');
densitySeries.setOptions({
  title: 'Sentinel-5P NRTI NO2',
  vAxis: {
    title: 'S5P N02'
  },
  lineWidth: 1,
  pointSize: 2,
  series: {
    0: {color: COLOR.xinxiang},
    1: {color: COLOR.milan},
    2: {color: COLOR.tehran}
  }
});

print(densitySeries);

Map.addLayer(xinjiang, {color: COLOR.xinjiang});
Map.addLayer(milan, {color: COLOR.milan});
Map.addLayer(tehran, {color: COLOR.tehran});
Map.setCenter(51.2, 35.6, 2);