Is the interval of SideTitles invalid for BarChart?

see BarChartSample9 in
https://zapp.run/edit/flutter-zphk0684phl0?entry=lib/main.dart&file=lib/bar_chart_sample9.dart


```dart
BarChartData(
titlesData: FlTitlesData(
bottomTitles: AxisTitles(
sideTitles: SideTitles(
showTitles: true,
interval: 3,
getTitlesWidget: (value, meta) {
return Text('$value');
},
))),
```

Start with the BarChartSample9 entry point in lib/bar_chart_sample9.dart and reproduce the behavior in the linked Zapp example using SideTitles with interval: 3. Trace how the bar chart handles that interval and determine whether the behavior is valid or needs a code or documentation change; done means the interval behavior is confirmed and the issue is resolved.

Written by the indexing model from the issue text.