I was able to reproduce your scenario in VS 2010 with SL version 4 and everything works fine to me.
You can find my example attached.
As I see from your picture X axis labels are overlapping and are hardly to read. What I suggest you is to rotate the axis labels to 90 degrees for example and set Layout Mode property to Inside so that ticks match labels. For more information about these properties take a look at our help topic - http://www.telerik.com/help/silverlight/radchart-features-axes-x-axis.html
Here is how you can set the mentioned values to the properties:
RadChart1.DefaultView.ChartArea.AxisX.LabelRotationAngle = 90;
RadChart1.DefaultView.ChartArea.AxisX.LayoutMode = AxisLayoutMode.Inside;
Besides if you are using XValue property to set X axis values the RadChart will automatically calculate how much of the values to show. You can find more information about this property in our help articles - http://www.telerik.com/help/silverlight/radchart-populating-with-data-creating-chart-in-code-behind.html
I hope this helps.
the Telerik team
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