BarChart Tooltip animation lags behind on fast value changes

**Describe the bug**
BarChart Tooltip animation lags behind on fast value changes. The data is provided with a simple provider

```
touchTooltipData: BarTouchTooltipData(
tooltipBgColor: Colors.transparent,
tooltipBottomMargin: -10,
getTooltipItem: (
BarChartGroupData group,
int groupIndex,
BarChartRodData rod,
int rodIndex,
) {
String display;
if (groupIndex == 0 && riskLabel != null) {
display = riskLabel;
} else {
display = StringUtils.format(rod.y) + '%';
}
return BarTooltipItem(
display,
const TextStyle(
color: Colors.black,
),
);
},
),
```

**Screenshots**

https://user-images.githubusercontent.com/54837156/113004403-dffb7f80-9173-11eb-8938-9a8e9f353bcf.mov

**Versions**
[✓] Flutter (Channel stable, 2.0.3, on Mac OS X 10.15.7 19H524 darwin-x64, locale de-DE)
fl_chart: ^0.20.1

Start by reproducing the BarChart tooltip with the supplied touchTooltipData and fast value changes on Flutter 2.0.3 with fl_chart 0.20.1. Compare the tooltip animation with the changing data; done means the tooltip keeps up without visibly lagging behind.

Written by the indexing model from the issue text.