I've tried to make a custom version of a BarChart. On a first try I've extended BarChart, but this was a bad idea. So I thought to make it better and take XYChart to extend. After that I copied the source from BarChart to my custom chart class (in another package than javafx.scene.chart). And then I get a lot of errors in Netbeans, that was ok. Now I've tried to solve these errors in my custom chart impl, but a lot of code is package protected in XYChart / XYChart.Series / XYChart.Data that should be public to have a base class.

As I've seen some code is package protected but in the comment is the meaning the opposite. Like this one in XYChart.Data:

  /**
* The current displayed data value plotted on the X axis. This may be the same as xValue or different. It is
* used by XYChart to animate the xValue from the old value to the new value. This is what you should plot
* in any custom XYChart implementations. Some XYChart chart implementations such as LineChart also use this
* to animate when data is added or removed.
*/
private ObjectProperty<X> currentX = new SimpleObjectProperty<X>(this, "currentX");
final X getCurrentX() { return currentX.get(); }
final void setCurrentX(X value) { currentX.set(value); }
final ObjectProperty<X> currentXProperty() { return currentX; }

And in XYChart you will wonder how to get the size of displayed Series List? There is a method

/**
* Computes the size of series linked list
* @return size of series linked list
*/
int getSeriesSize() {
   return displayedSeries.size();
}

But again it is package protected.

So there is much more like that, so how should one extend XYChart to make a custom two axis chart diagramm with all that issues?