code: public void setCustomCategoryLabels(Map<Object, Object> customCategoryLabels) {
  
      // get the first series
      AxesChartSeriesCategory axesChartSeries = getSeriesMap().values().iterator().next();
      // get the first categories, could be Number Date or String
      List<?> categories = (List<?>) axesChartSeries.getXData();
  
      Map<Double, Object> axisTickValueLabelMap = new LinkedHashMap<Double, Object>();
      for (Entry<Object, Object> entry : customCategoryLabels.entrySet()) {
        int index = categories.indexOf(entry.getKey());
        if (index == -1) {
          throw new IllegalArgumentException("Could not find category index for " + entry.getKey());
        }
        axisTickValueLabelMap.put((double) index, entry.getValue());
      }
  
      setXAxisLabelOverrideMap(axisTickValueLabelMap);
    }
docs: Set custom X-Axis category labels
  
  @param customCategoryLabels Map containing category name -> label mappings