You should be able to remove the whitespace at the top with the described code as it is controlled by the top margin value of the plot area. Attached is a sample application of our observations.
Alternatively, for the Q1 2008 release we introduced new feature for RadChart -- AutoLayout. The AutoLayout functionality instructs the control to perform layout
arrangement automatically for optimal utilization of the whole chart
area and ensures that all chart
elements are fully readable and do not overlap with one another. To
enable this feature set the RadChart.AutoLayout
You can find more information here
and online example here
Hope this helps.
the Telerik team