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