BarChar sample source

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**Describe the bug**
In this sample source: https://github.com/imaNNeoFighT/fl_chart/blob/master/example/lib/bar_chart/samples/bar_chart_sample3.dart#L160

The barchart labels are duplicated:

**To Reproduce**
These should be 4 and 5 respectively

**Versions**
- which version of the Flutter are you using? 3.0.1
- which version of the FlChart are you using? 0.55.1

I can add a pull request changing the numbers, however I wasn't sure that this was left on purpose for us to learn how to use the library, at first I thought this was a bug.

Open example/lib/bar_chart/samples/bar_chart_sample3.dart around line 160 and inspect the bar-chart label values shown in the issue. Confirm which labels are duplicated and compare them with the expected values 4 and 5. Update the sample and run the relevant Flutter example or tests to verify that the labels render correctly.

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