Time series analysis of NDVI using Google Earth Engine

Question

I want to create a chart of annual NDVI for a specific region. The output should be one NDVI value for that region for each year. I tried the following code and so far was able to get an image collection that consists of annual NDVI image. But I am not sure how can I chart OR export the data? The "chart" part of the code is showing error.

  //Importing image and geometry:
var l8 = ee.ImageCollection("LANDSAT/LC08/C01/T1_TOA"),
    geometry = /* color: #d63000 */ee.Geometry.Polygon(
        [[[-80.92489760146748, 25.433457120928352],
          [-80.64474623427998, 25.488013471687964],
          [-80.57882826552998, 25.710940372707608],
          [-81.02377455459248, 25.770317250349557],
          [-80.95236342177998, 25.552457242621447]]]);

//Filtering date, polygon, and cloudiness
    var image = l8.filterDate ('2014-01-01', '2018-12-31')
                .filterBounds (geometry)
                .filterMetadata ('CLOUD_COVER', 'less_than', 20);

    //NDVI calculation:
    var ndvi_func = function (i) {
      var ndvi = i.normalizedDifference (['B5', 'B4']).rename ('NDVI')
      return i.addBands(ndvi);
    }

    var image_ndvi = image.map(ndvi_func);

    //Calculating year wise NDVI
    var year = ee.List.sequence(2014,2018);
    var year_func = function(y){
      var range = ee.Filter.calendarRange (y, y, 'year');
      return image_ndvi.select('NDVI').filter(range).mean().set ('Year', y)
    };
    var yearwise_ndvi = ee.ImageCollection(year.map(year_func));
    print (yearwise_ndvi);
    Map.addLayer (yearwise_ndvi)



//Creating time-series chart:
        var chart = ui.Chart.image.series ({
          imageCollection: yearwise_ndvi,
          region: geometry,
          reducer: ee.Reducer.mean(),
          scale: 30
        }).setOptions ({title: "NDVI over time",
          hAxis: {title: 'Time of the year', format: 'year'}
        });

        print (chart);

Answer 1

ui.Chart.image.series defaults to plotting system:time_start on the x-axis, which your dataset does not contain. You can use the optional argument xProperty to specify another parameter to use as x-axis, in your case Year:

//Creating time-series chart:
var chart = ui.Chart.image.series ({
  imageCollection: yearwise_ndvi,
  region: geometry,
  reducer: ee.Reducer.mean(),
  scale: 30,
  xProperty: 'Year'
}).setOptions ({title: "NDVI over time",
  hAxis: {title: 'Time of the year', format: 'year'}
});