Changing the Offset for All Slices
To change the offset for all Pie slices, use the following code:
chart.DataContext = new double[] { 1, 2, 3 };
chart.ChartType = ChartType.Pie;
chart.Loaded += (s, e) => ((BasePieRenderer)chart.Data.Renderer).Offset = 0.1;
Also it's possible to change offset for the specific slice but it requires to change the position manually in PlotElementLoaded event.