Offset argument contained a NaN value, happens when maxPrice and minPrice are equal

```Offset argument contained a NaN value.
'dart:ui/painting.dart':
Failed assertion: line 52 pos 10: '<optimized out>'
```

ig this happens when maxPrice and minPrice are the same in a set of data.
this happens in fitPrice function
```
  double fitPrice(double y) =>
      priceHeight * (maxPrice - y) / (maxPrice - minPrice);

```

and thanks for the amazing package!
