code: public Options setxAxis(final Axis xAxis) {
          this.xAxis = new ArrayList<Axis>();
          this.xAxis.add(xAxis);
          return this;
      }
docs: Sets the single xAxis. If you need to define more than one xAxis use
  {@link #addxAxis(Axis)}. If there are multiple Axes defined when calling
  this method, only the specified axis will be defined afterwards.
  
  @param xAxis the single xAxis of the chart
  @return the {@link Options} object for chaining