Run the attached app. It creates BubbleChart using CategoryAxis as it xAxis.

ClassCastException is thrown:
Caused by: 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:687)
at javafx.scene.chart.Chart$1.layoutChildren(Chart.java:81)
at javafx.scene.Parent.layout(Parent.java:1002)
at javafx.scene.Scene.layoutDirtyRoots(Scene.java:495)
at javafx.scene.Scene.doLayoutPass(Scene.java:468)
at javafx.scene.Scene.preferredSize(Scene.java:1244)
at javafx.scene.Scene.impl_initPeer(Scene.java:631)
at javafx.stage.Window$10.invalidated(Window.java:714)
at javafx.beans.property.BooleanPropertyBase.markInvalid(BooleanPropertyBase.java:129)
at javafx.beans.property.BooleanPropertyBase.set(BooleanPropertyBase.java:163)
at javafx.stage.Window.setShowing(Window.java:778)
at javafx.stage.Window.show(Window.java:793)
at javafx.stage.Stage.show(Stage.java:229)
at Bug.start(Bug.java:48)

BubbleChart javadoc doesn't indicate there are constraints on axes.