rangeValue method
Implementation
@override
num rangeValue(DynamicData domainData) {
num index = domain.indexOf(domainData.data);
if (index == -1) {
return double.nan;
}
index += 0.5; //居中
num diff = range.last - range.first;
num interval = diff / domain.length;
return range.first + index * interval;
}