The bubble chart probably makes sense only if the x-axis is a NumberAxis. Otherwise, the bubble radius is meaningless along the x-axis. However, it does not complain when I provide a CategoryAxis instead. During layouting it will run into a ClassCastException then (see below). From this point on the scene (in a JFXPanel) is frozen.

java.lang.ClassCastException: javafx.scene.chart.CategoryAxis cannot be cast to javafx.scene.chart.NumberAxis
at javafx.scene.chart.BubbleChart.layoutPlotChildren(BubbleChart.java:117)
at javafx.scene.chart.XYChart.layoutChartChildren(XYChart.java:688)
at javafx.scene.chart.Chart$1.layoutChildren(Chart.java:84)
at javafx.scene.Parent.layout(Parent.java:1018)
at javafx.scene.Scene.layoutDirtyRoots(Scene.java:513)
at javafx.scene.Scene.doLayoutPass(Scene.java:484)
at javafx.scene.Scene.access$3900(Scene.java:169)
at javafx.scene.Scene$ScenePulseListener.pulse(Scene.java:2199)
at com.sun.javafx.tk.Toolkit.firePulse(Toolkit.java:363)
at com.sun.javafx.tk.quantum.QuantumToolkit.pulse(QuantumToolkit.java:460)
at com.sun.javafx.tk.quantum.QuantumToolkit$9.run(QuantumToolkit.java:329)
at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
at com.sun.glass.ui.win.WinApplication.access$100(WinApplication.java:29)
at com.sun.glass.ui.win.WinApplication$2$1.run(WinApplication.java:67)
at java.lang.Thread.run(Unknown Source)

I think this is a minor priority bug. Situation could be improved easily by throwing IllegalArgumentException when trying to set a non-number x-axis. Also, it would be good to document this requirement.