code: @Pure
  	public DoubleProperty secondAxisExtentProperty() {
  		if (this.extentS == null) {
  			this.extentS = new SimpleDoubleProperty(this, MathFXAttributeNames.SECOND_AXIS_EXTENT) {
  				@Override
  				protected void invalidated() {
  					if (get() < 0.) {
  						set(0.);
  					}
  				}
  			};
  		}
  		return this.extentS;
  	}
docs: Replies the property for the extent of the second axis.
  
  @return the secondAxisExtent property.