public Options setxAxis(final Axis xAxis) {
        this.xAxis = new ArrayList<Axis>();
        this.xAxis.add(xAxis);
        return this;
    }
public Options setxAxis ( final Axis xAxis ) { this . xAxis = new ArrayList < Axis > ( ) ; this . xAxis . add ( xAxis ) ; return this ; }
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