Use a single data set instead of one per activity kind

The reason is that multiple data sets will always be grouped.
If we add null values to fix the grouping issue, we will still have
space between the bars.

app/src/main/java/nodomain/freeyourgadget/gadgetbridge/activities/SleepChartActivity.java

@@ -118,7 +118,7 @@ public class SleepChartActivity extends Activity {
 
         YAxis y = mChart.getAxisLeft();
         y.setDrawGridLines(false);
-        y.setDrawLabels(false);
+//        y.setDrawLabels(false);
         y.setDrawTopYLabelEntry(false);
         y.setTextColor(Color.WHITE);