Double precision issue on dynamic min-max value

**Is your feature request related to a problem? Please describe.**
Wanna Add Widget on Ruler (Using Custom Ruler not good, since have to calculate interval(steps by myself). When i change min max, i face with Double precision error. ex: 3.0 + 3.0 = 7. (just example or sometimes its like this: 7.942123212),

**Describe the solution you'd like**
There are some packages that offer to customize the label.
```
Widget buildLabel(double value, int index) => MyCustomLabel(...);
```
Personally for calculating double values I use decimal package. So im try to use that here. but cannot.

I don't think its that difficult to add such feature.
adding callback property in RulerStyle might be good solution. and if that callback is null, use default Widget.