invertExtent method
Returns the extent of values in the domain that map to the given range value.
Implementation
(double, double) invertExtent(R value) {
final i = _range.indexOf(value);
if (i < 0) return (double.nan, double.nan);
final lo = i > 0 ? _thresholds[i - 1] : _domain.first;
final hi = i < _thresholds.length ? _thresholds[i] : _domain.last;
return (lo, hi);
}