Add range selection to line chart

It would be an amazing option to be able to select a range in a line chart


I have implemented it like this now:
```dart
touchCallback: (LineTouchResponse touchResponse) {
if (touchResponse.lineBarSpots.first.x != minX) {
setState(() {
startRange = 1.0 * touchResponse.lineBarSpots.first.x;
endRange = startRange + 1;
});
}
},
```

And change the gradient colors stops relative to startRange and endRange.

The only problem I have now is that the spot is at the left border of the range, so the touchSpotThreshold it is not relative to the center of the range so the touch is displaced.
I need to put the spot in the borders and not in the centers so I can add vertical border lines to the selected range

No files or tests are named; start from the line chart's touchCallback and the supplied range-selection experiment, then inspect the existing touch-spot and gradient behavior. Done means a user can select a range with border-aligned spots or vertical boundaries and touch handling that is not displaced.

Written by the indexing model from the issue text.