The reported behavior is caused by the fact that on each layout resize the chart's internal logic checks if the bottom edge of each label is lower than the bottom of the plot area. If so, the label is not added in the chart and therefore it is not visible. In other words if there is not enough space for the label to be displayed correctly centered to the bar, it is removed from the plot area.
To resolve this you can set smaller FontSize for the labels through the SeriesItemLabelStyle.
Another approach could be to restrict the chart's height with its MinHeight property.
Please, keep in mind that RadChart is our old charting component which has many known issues and limitations. This is why we decided to create new charting suite. The new RadChartView
consists of few chart controls that can be used depending on your scenario. Furthermore, RadChartView has better implementation and improved performance compared to RadChart. In addition the new components are very flexible and easier to set up. You can read more about the differences between the components in the RadChart vs. RadChartView
If you just start using a chart or its integration is not very complex I recommend you to try RadChartView.
Please let me know if you need any further assistance.
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