Make line chart with values and dates - mpandroidchart

Question

In my app i use ios-charts library (swift alternative of MPAndroidChart).
All i need is to display line chart with dates and values.
Right now i use this function to display chart
func setChart(dataPoints: [String], values: [Double]) {
var dataEntries: [ChartDataEntry] = []
for i in 0..<dataPoints.count {
let dataEntry = ChartDataEntry(value: values[i], xIndex: i)
dataEntries.append(dataEntry)
}
let lineChartDataSet = LineChartDataSet(yVals: dataEntries, label: "Items count")
let lineChartData = LineChartData(xVals: dataPoints, dataSet: lineChartDataSet)
dateChartView.data = lineChartData
}
And this is my data:
xItems = ["27.05", "03.06", "17.07", "19.09", "20.09"] //String
let unitsSold = [25.0, 30.0, 45.0, 60.0, 20.0] //Double
But as you can see - xItems are dates in "dd.mm" format. As they are strings they have same paddings between each other. I want them to be more accurate with real dates. For example 19.09 and 20.09 should be very close. I know that i should match each day with some number in order to accomplish it. But i don't know what to do next - how i can adjust x labels margins?
UPDATE
After small research where i found out that many developers had asked about this feature but nothing happened - for my case i found very interesting alternative to this library in Swift - PNChart. It is easy to use, it solves my problem.

Answer 1

The easiest solution will be to loop through your data and add a ChartDataEntry with a value of 0 and a corresponding label for each missing date.
In response to the question in the comments here is a screenshot from one of my applications where I am filling in date gaps with 0 values:
In my case I wanted the 0 values rather than an averaged line from data point to data point as it clearly indicates there is no data on the days skipped (8/11 for instance).
From #Philipp Jahoda's comments it sounds like you could skip the 0 value entries and just index the data you have to the correct labels.
I modified the MPAndroidChart example program to skip a few data points and this is the result:
As #Philipp Jahoda mentioned in the comments the chart handles missing Entry by just connecting to the next data point. From the code below you can see that I am generating x values (labels) for the entire data set but skipping y values (data points) for index 11 - 29 which is what you want. The only thing remaining would be to handle the x labels as it sounds like you don't want 15, 20, and 25 in my example to show up.
ArrayList<String> xVals = new ArrayList<String>();
for (int i = 0; i < count; i++) {
xVals.add((i) + "");
}
ArrayList<Entry> yVals = new ArrayList<Entry>();
for (int i = 0; i < count; i++) {
if (i > 10 && i < 30) {
continue;
}
float mult = (range + 1);
float val = (float) (Math.random() * mult) + 3;// + (float)
// ((mult *
// 0.1) / 10);
yVals.add(new Entry(val, i));
}

Answer 2

What I did is fully feed the dates for x data even no y data for it, and just not add the data entry for the specific xIndex, then it will not draw the y value for the xIndex to achieve what you want, this is the easiest way since you just write a for loop and continue if you detect no y value there.
I don't suggest use 0 or nan, since if it is a line chart, it will connect the 0 data or bad things will happen for nan. You might want to break the lines, but again ios-charts does not support it yet (I also asked a feature for this), you need to write your own code to break the line, or you can live with connecting the 0 data or just connect to the next valid data.
The down side is it may has performance drop since many xIndex there, but I tried ~1000 and it is acceptable. I already asked for such feature a long time ago, but it took lot of time to think about it.

Answer 3

Here's a function I wrote based on Wingzero's answer (I pass NaNs for the entries in the values array that are empty) :
func populateLineChartView(lineChartView: LineChartView, labels: [String], values: [Float]) {
var dataEntries: [ChartDataEntry] = []
for i in 0..<labels.count {
if !values[i].isNaN {
let dataEntry = ChartDataEntry(value: Double(values[i]), xIndex: i)
dataEntries.append(dataEntry)
}
}
let lineChartDataSet = LineChartDataSet(yVals: dataEntries, label: "Label")
let lineChartData = LineChartData(xVals: labels, dataSet: lineChartDataSet)
lineChartView.data = lineChartData
}

Answer 4

The solution which worked for me is splitting Linedataset into 2 Linedatasets. First would hold yvals till empty space and second after emptyspace.
//create 2 LineDataSets. set1- till empty space set2 after empty space
set1 = new LineDataSet(yVals1, "DataSet 1");
set2= new LineDataSet(yVals2,"DataSet 1");
//load datasets into datasets array
ArrayList<ILineDataSet> dataSets = new ArrayList<ILineDataSet>();
dataSets.add(set1);
dataSets.add(set2);
//create a data object with the datasets
LineData data = new LineData(xVals, dataSets);
// set data
mChart.setData(data);