I believe that you have used different approach in placing the legend on top than the out-of-the-box solution, because when you set this:
radChart.DefaultView.ChartLegendPosition = Dock.Top;
The chart legend automatically arranges its items horizontally. Note that for this to work you have to use the DefaultLayout of , and not a custom one.
I hope this is the solution you are searching for. You can check this
article for more information.
the Telerik team