Setting Data

If you want to add values (data) to the chart, it has to be done via the

    public void setData(ChartData data) { ... }

method. The baseclass ChartData class encapsulates all data and information that is needed for the chart during rendering. For each type of chart, a different subclass of ChartData (e.g. LineData) exists that should be used for setting data for the chart. In the constructor, you can hand over an ArrayList<? extends DataSet> as the values to display, and an additional ArrayList of String that will describe the labels on the x-axis. Example with the class LineData (extends ChartData), which is used for adding data to a LineChart:

    // this is just one of many constructors
    public LineData(ArrayList<String> xVals, ArrayList<LineDataSet> sets) { ... }

So, what is a DataSet and why do you need it? That is actually pretty simple. One DataSet object represents a group of entries (datatype Entry) inside the chart that belong together. It is designed to logically separate different groups of values in the chart. For each type of chart, a differnt object that extends DataSet (e.g. LineDataSet) exists that allows specific styling.

As an example, you might want to display the quarterly revenue of two different companies over one year in a LineChart. In that case, it would be recommended to create two different LineDataSet objects, each containing four values (one for each quarter). As an ArrayList to describe the labels on the x-axis, you would simply provide the four Strings "1.Q", "2.Q", "3.Q", "4.Q".

Of course, it is also possible to provide just one LineDataSet object containing all 8 values for the two companys.

So how to setup a LineDataSet object?

    public LineDataSet(ArrayList<Entry> yVals, String label) { ... }

When looking at the constructor (different constructors are available), it is visible that the LineDataSet needs an ArrayList of type Entry and a String used to describe the LineDataSet and as a label used for the Legend. Furthermore this label can be used to find the LineDataSet amongst other LineDataSet objects in the LineData object.

The ArrayList of type Entry encapsulates all values of the chart. A Entry object is an additional wrapper around a value and holds the value itself, and it's position on the x-axis (the index inside the ArrayList of String of the LineData object the value is mapped to):

    public Entry(float val, int xIndex) { ... }

Putting it all together (example of two companies with quarterly revenue over one year):

At first, create the lists of type Entry that will hold your values:

    ArrayList<Entry> valsComp1 = new ArrayList<Entry>();
    ArrayList<Entry> valsComp2 = new ArrayList<Entry>();

Then, fill the lists with Entry objects. Make sure the entry objects contain the correct indices to the x-axis. (of course, a loop can be used here, in that case, the counter variable of the loop could be the index on the x-axis).

    Entry c1e1 = new Entry(100.000f, 0); // 0 == quarter 1
    valsComp1.add(c1e1);
    Entry c1e2 = new Entry(50.000f, 1); // 1 == quarter 2 ...
    valsComp1.add(c1e2);
    // and so on ...
    
    Entry c2e1 = new Entry(120.000f, 0); // 0 == quarter 1
    valsComp2.add(c2e1);
    Entry c2e2 = new Entry(110.000f, 1); // 1 == quarter 2 ...
    valsComp2.add(c2e2);
    //...

Now that we have our lists of Entry objects, the LineDataSet objects can be created:

    LineDataSet setComp1 = new LineDataSet(valsComp1, "Company 1");
    LineDataSet setComp2 = new LineDataSet(valsComp2, "Company 2");

Last but not least, we create a list of DataSets and a list of x-axis entries and build our ChartData object:

    ArrayList<LineDataSet> dataSets = new ArrayList<LineDataSet>();
    dataSets.add(setComp1);
    dataSets.add(setComp2);
    
    ArrayList<String> xVals = new ArrayList<String>();
    xVals.add("1.Q"); xVals.add("2.Q"); xVals.add("3.Q"); xVals.add("4.Q"); 
    
    LineData data = new LineData(xVals, dataSets);
    mLineChart.setData(data);
    mLineChart.invalidate(); // refresh

After calling invalidate() the chart is refreshed and the provided data is drawn.