code: public Options addxAxis(final Axis xAxis) {
          if (this.getxAxis() == null) {
              this.setxAxis(new ArrayList<Axis>());
          }
          this.getxAxis().add(xAxis);
          return this;
      }
docs: Adds an XAxis to the chart. You can use {@link #setxAxis(Axis)} if you want
  to define a single axis only.
  
  @param xAxis the XAxis to add.
  @return the {@link Options} object for chaining.