toBar method
Maps platter coordinates (the bar's layout) to where they are drawn.
Implementation
Offset toBar(Offset p) {
final Offset d = platterDrift;
return Offset(
_size.width / 2 + d.dx + (p.dx - _size.width / 2) * platterScaleX,
_size.height / 2 + d.dy + (p.dy - _size.height / 2) * platterScaleY,
);
}