The result you describe is expected, because in the calculations for the AutoRange functionality, the null values are treated as 0. In this case, I would recommend that after the chart is databound, you check all data points in the DataSeries collection of the chart and remove those series that contain only null points. For example:
sender, ChartDataBoundEventArgs e)
> pointPredicate = point => point.YValue == 0;
var dataSeries = RadChart1.DefaultView.ChartArea.DataSeries.Where(item => item.ToList().All(pointPredicate));
series.Definition.Visibility = SeriesVisibility.Collapsed;
The other approach that you can use, it not to use the AutoRange option and define the values of the YAxis manually. Please refer to our online documentation for information how to do this - http://www.telerik.com/help/silverlight/radchart-features-axes-overview.html
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