try use this method.
public static void setAxisTitle(XSSFChart chart, int axisIdx, String title) {
CTValAx valAx = chart.getCTChart().getPlotArea().getValAxArray(axisIdx);
CTTitle ctTitle = valAx.addNewTitle();
ctTitle.addNewLayout();
ctTitle.addNewOverlay().setVal(false);
CTTextBody rich = ctTitle.addNewTx().addNewRich();
rich.addNewBodyPr();
rich.addNewLstStyle();
CTTextParagraph p = rich.addNewP();
p.addNewPPr().addNewDefRPr();
p.addNewR().setT(title);
p.addNewEndParaRPr();
}