For those like me who want to achieve this but keep the original library, here is a simple solution inspired by @fgueli's modifications. This applies for one break line only (add "\n" in your labels) but you can easily adapt it to your needs.
Subclass XAxisRenderer
public class CustomXAxisRenderer extends XAxisRenderer {
public CustomXAxisRenderer(ViewPortHandler viewPortHandler, XAxis xAxis, Transformer trans) {
super(viewPortHandler, xAxis, trans);
}
@Override
protected void drawLabel(Canvas c, String formattedLabel, float x, float y, MPPointF anchor, float angleDegrees) {
String line[] = formattedLabel.split("\n");
Utils.drawXAxisValue(c, line[0], x, y, mAxisLabelPaint, anchor, angleDegrees);
Utils.drawXAxisValue(c, line[1], x + mAxisLabelPaint.getTextSize(), y + mAxisLabelPaint.getTextSize(), mAxisLabelPaint, anchor, angleDegrees);
}
}
Set this renderer on the desired chart
lineChart.setXAxisRenderer(new CustomXAxisRenderer(lineChart.getViewPortHandler(), lineChart.getXAxis(), lineChart.getTransformer(YAxis.AxisDependency.LEFT)));