The trouble with Charting with Google Earth Engine - charts

Question

In Google Earth Engine, I want to get the NDVI index from several images of the Sentinel 2 satellite with different dates, then I will estimate other parameters from this index. To do this, I need to convert the resulting NDVI images to an image collection. But to chart it, it gives the following error:
Error generating chart: No features contain non-null values of "system:time_start".
It seems that when converting to a collection image, the temporal information of the images is lost. with this condition, how can I fix it?
Link to the code: https://code.earthengine.google.com/47cd9e7f65b143242ebb238d136bf760
Code:
var sentinel1 = ee.Image('COPERNICUS/S2_SR/20181214T072311_20181214T072733_T39SVV');
var sentinel2 = ee.Image('COPERNICUS/S2_SR/20181219T072319_20181219T072610_T39SVV');
var sentinel3 = ee.Image('COPERNICUS/S2_SR/20181224T072311_20181224T072313_T39SVV');
var ndvi1 = sentinel1.normalizedDifference(['B8','B4']);
var ndvi2 = sentinel2.normalizedDifference(['B8','B4']);
var ndvi3 = sentinel3.normalizedDifference(['B8','B4']);
var NDVI_COL = ee.ImageCollection.fromImages([ndvi1, ndvi2, ndvi3]);
var chart = ui.Chart.image.series(
NDVI_COL, geometry, ee.Reducer.mean(),10,'system:time_start');
print(chart);

Answer 1

Try this code
Make a collection of selected images, then plot
var sentinel1 = ee.Image('COPERNICUS/S2_SR/20181214T072311_20181214T072733_T39SVV');
var sentinel2 = ee.Image('COPERNICUS/S2_SR/20181219T072319_20181219T072610_T39SVV');
var sentinel3 = ee.Image('COPERNICUS/S2_SR/20181224T072311_20181224T072313_T39SVV');
var S2 = ee.ImageCollection([sentinel1, sentinel2, sentinel3])
.filterBounds(geometry)
.map(function(image){return image.clip(geometry)});
print('collection of Selected Images to plot: ', S2);
var addNDVI = function(image) {
var ndvi = image.normalizedDifference(['B8', 'B4']).rename('NDVI');
return image.addBands(ndvi);
};
var S2_NDVI = S2.map(addNDVI);
var NDVI_S2 = ee.ImageCollection(S2_NDVI.select(["NDVI"], ["NDVI"]));
var chart =
ui.Chart.image
.seriesByRegion({
imageCollection: NDVI_S2,
band: 'NDVI',
regions: geometry,
reducer: ee.Reducer.mean(),
scale: 10,
xProperty: 'system:time_start'
})
.setOptions({
title: 'Average NDVI Value by Date',
hAxis: {title: 'Date', titleTextStyle: {italic: false, bold: true}},
vAxis: {
title: 'NDVI',
titleTextStyle: {italic: false, bold: true}
},
});
print(chart);