How to format values inside MPAndroidChart?

If all you want is to change the number of decimal places on the values, the DefaultValueFormatter will suffice.

pieDataSet.setDefaultValueFormatter(new DefaultValueFormatter(digits = 1))
//where digits is the number of decimal places

The example above will format 88.65 as 88.7

Have a look at the IValueFormatter interface provided by the library. With that interface, you can completely customize what gets displayed on the chart based on your own logic.

Usage:

chart.setValueFormatter(new YourValueFormatter());
YLabels yl = chart.getYLabels();
yl.setFormatter(new YourValueFormatter());

UPDATE (for versions 2.0.0+ of this [library][2]):

Now, a ValueFormatter can be set for each DataSet separately, or the same ValueFormatter can be set for the whole data object containig all DataSets. Furthermore, the YLabels class is now called YAxis.

Example:

// set for whole data object (individual DataSets also possible)
LineData data = new LineData(...);
data.setValueFormatter(new YourValueFormatter());

// YLabels are now called YAxis
YAxis yAxis = mChart.getAxisLeft(); // get the left or right axis
yAxis.setValueFormatter(new YourAxisValueFormatter());

UPDATE (for versions 3.0.0+ of this [library][2]):

The interfaces for formatting have been renamed and extended in their functionality. Now, the IAxisValueFormatter can be used to format values of both XAxis and YAxis. The IValueFormatter interface is used to customize chart values.

Link to the ValueFormatter documentation.