onAttach method
Handles any setup of the renderer that needs to be deferred until it is attached to a chart.
Implementation
@override
void onAttach(BaseChart<D> chart) {
if (chart is! CartesianChart<D>) {
throw ArgumentError(
'SymbolAnnotationRenderer can only be attached to a CartesianChart<D>');
}
_chart = chart;
// Only vertical rendering is supported by this behavior.
assert(_chart.vertical);
super.onAttach(chart);
_chart.addView(this);
}