An IAxisTitle extends IChartTitle and provides access to the text and formatting of an axis title. Use IAxis.getAxisTitle() to obtain the title object for a chart axis.

 worksheet.getRange("A1:B3").setValue(new Object[][] {
     {"Month", "Sales"},
     {"Jan", 100},
     {"Feb", 120}
 });
 IChart chart = worksheet.getShapes().addChart(ChartType.ColumnClustered, 20, 20, 300, 200).getChart();
 chart.getSeriesCollection().add(worksheet.getRange("B2:B3"), RowCol.Columns);
 IAxisTitle axisTitle = chart.getAxes().item(AxisType.Value).getAxisTitle();
 axisTitle.setText("Sales");