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