Error generating chart: No features contain non-null values of "system:time_start" Create NO2 chart with unit ton/m2 in GEE Sentinel-5p

Question

I'm using the below code to create NO2(Sentinel-5p) time series charts for my region of interest by mean reduction. However, upon charting, I get an error message

Error generating chart: No features contain non-null values of "system:time_start"

Please, somebody advise me how to fix this error?

Here is my code:

    var geometry = 
        /* color: #d63000 */
        /* displayProperties: [
          {
            "type": "rectangle"
          }
        ] */
        ee.Geometry.Polygon(
            [[[112.19326448481443, 37.22101939910812],
              [112.19326448481443, 36.467942373936516],
              [113.19250536005858, 36.467942373936516],
              [113.19250536005858, 37.22101939910812]]], null, false),
    
    
    // Create a FeatureCollection from the list and print it.
    var geometry = ee.FeatureCollection(geometry);
    print(geometry);
    Map.centerObject(geometry);
    Map.addLayer(geometry);
    
//Importing image and geometry:
    var collection = ee.ImageCollection('COPERNICUS/S5P/NRTI/L3_NO2')
      .select('NO2_column_number_density')
      .filterDate('2019-01-01', '2019-01-28')
      .filterBounds(geometry);
    
    // calculation mol/m^2 to ton/m^2.  
    var mol_to_ton = collection.map(function (image) {
      return image.multiply(0.000046);
    });
    
    
    //Visualization parameters
    var band_viz = {
      min: 0,
      max: 0.00000002,
      palette: ['black', 'blue', 'purple', 'cyan', 'green', 'yellow', 'red']
    };
    
    
    Map.addLayer(mol_to_ton.mean(), band_viz, 'S5P N02_ton');
    
    // Create and print the chart.
    print(ui.Chart.image.series(mol_to_ton.mean(), geometry, ee.Reducer.mean(), 1113, 'system:time_start'));

Answer 1

There are 2 things:

1. ui.Chart.image.series takes an ImageCollection as first argument (https://developers.google.com/earth-engine/apidocs/ui-chart-image-series) and you are passing an image (mol_to_ton.mean())
2. When you perform a mathematical operation over an Image it "looses" its properties, so you have to add them again.

I will write here the relevant changes, and will paste a link to the complete working code.

// calculation mol/m^2 to ton/m^2.  
var mol_to_ton = collection.map(function (image) {
  return image.multiply(0.000046).set('system:time_start', image.date().millis());
});

and

print(ui.Chart.image.series(mol_to_ton, geometry, ee.Reducer.mean(), 1113, 'system:time_start'));

https://code.earthengine.google.com/034d11cb81b3d46d81f2466188ac528e