Chart number of images within AOI aggregated by month in GEE

Question

My goal is to chart the number of Sentinel-2 images within AOI aggregated by month on a histogram. Currently I use ui.Chart.feature.histogram and plot by property 'month' that I extracted from image 'system:time_start'. This works for intervals within 1 year, but if I increase the interval it starts agregating corresponding months for different years.

But the goal is to have an X-axis representing Years and Months like on time series chart (below) and Y-axis represent number of images aggregated by month, so how do I do it?

Here is my current implementation:

// ================================ VARS & PARAMS ==========================================

// data

var s2_coll = ee.ImageCollection('COPERNICUS/S2');


// aoi

var aoi = ee.Geometry.Polygon(
  [[[12.073281620012267, 42.652666751867734],
    [12.073281620012267, 42.60214521751987],
    [12.150185916887267, 42.60214521751987],
    [12.150185916887267, 42.652666751867734]]], null, false);

// dates 

var year_start = 2019;
var year_end = null; // DO NOT FUNCTION YET

var month_start = 1;
var month_end = 12;

// styling and display

var chart_options = {
    title: 'S2 imagery number',
    hAxis: {title: 'Month'},
    vAxis: {title: 'Image count'},
    colors: ['orange']
};

// misc


// ================================ FUNCTIONS ==============================================

var metadata_collector = function(img) {
  var month  = img.date().format("M"); // get month
  return img.set('month', ee.Number.parse(month)); // 
};

// ================================ IMPLEMENTATION =========================================


// preliminary filtering collection by aoi and dates
var s2_coll_filt = s2_coll
    .filterBounds(aoi)
    .filter(ee.Filter.calendarRange(year_start, year_end, 'year'))
    .filter(ee.Filter.calendarRange(month_start, month_end, 'month'));


var s2_coll_filt = s2_coll_filt.map(metadata_collector);

var s2_fc = ee.FeatureCollection(s2_coll_filt);

var num_scn_total = ee.FeatureCollection(s2_coll_filt).size();


var txt_total_scns = 'Total number of scenes within AOI is: ';
var info_text = ee.String(txt_total_scns).cat(num_scn_total);

// ================================ VISUALIZATION ==========================================

// map
Map.centerObject(aoi);

// console
info_text.evaluate(function(result){
  print(result);
});


var histogram = ui.Chart.feature.histogram({
    features: s2_fc,
    // property: 'system:time_start',
    property: 'month',
    minBucketWidth: 1
}).setOptions(chart_options);


print(histogram);

Code Editor script

Answer 1

You can do that fairly easy. I made something similar (or add this), but then combining Landsat and Sentinel data and aggregated that over multiple years (ignoring months). So I used part of that code, and part of your provided code to produce what you are trying to achieve.

// use function to get imagery count
var FC = metadata_collector(s2_coll, aoi, year_start, year_end, month_start, month_end);

// make the graph
var chart = ui.Chart.feature.byFeature(FC, 'system:time_start', ['S2'])
                  .setChartType('ColumnChart').setOptions(chart_options);
print(chart);

Where the function was defined as:

function metadata_collector(data, geometry, startYear, endYear, startMonth, endMonth) {

  // Map over the years 
  var listYears = ee.List.sequence(startYear, endYear, 1);
  var monthList = ee.List.sequence(startMonth, endMonth, 1);

  // map over the years, get collection size per year and return as feature
  var feats = ee.FeatureCollection(listYears.map(function(year){
      // map over the month
      var byMonth = monthList.map(function(month){
        // set start and end date from the year
        var start = ee.Date.fromYMD(year, month, 1);
        var end = start.advance(1, 'month');

        // filter the collection
        var S2_filt = data.filterBounds(geometry).filterDate(start, end);

        // daily moaic sentinel-2 images
        var S2_size = dailyMosaics(S2_filt); // see function in link

        // Return the size in a feature
        var feat = ee.Feature(geometry, {year: year, 'system:time_start': start.millis(),
                      S2: S2_size}); // geometry added to allow exporting
        return feat;
    });
    return byMonth;
  }).flatten());

  return feats;
}

Full link code