If I inject a field LineChart<Number, Number> lineChart, it will throw a ClassCastException:

java.lang.ClassCastException: java.lang.Integer cannot be cast to java.lang.String
at javafx.scene.chart.CategoryAxis.invalidateRange(CategoryAxis.java:399)
at javafx.scene.chart.XYChart.updateAxisRange(XYChart.java:603)
at javafx.scene.chart.XYChart.layoutChartChildren(XYChart.java:620)
at javafx.scene.chart.Chart$1.layoutChildren(Chart.java:84)
at javafx.scene.Parent.layout(Parent.java:1018)

Works with LineChart<String, Number>.