registerTappable method
Tells the chart that this behavior responds to tap events.
This should only be called after behavior
has been attached to the chart
via addBehavior.
Implementation
void registerTappable(ChartBehavior<D> behavior) {
final role = behavior.role;
if (_behaviorRoleMap[role] == behavior &&
_behaviorTappableMap[role] != behavior) {
_behaviorTappableMap[role] = behavior;
}
}