Add the ability a user to be able to use a Custom DashStyle in the following scenario:

this.Chart.ShowLegend = true;
this.Chart.Series.Add(new LineSeries() { LegendTitle = "DDDD" });
this.Chart.ChartElement.LegendElement.Items[0].Element.BorderDashStyle = System.Drawing.Drawing2D.DashStyle.Custom;

for (int i = 0; i < 10; i++)
{
    this.Chart.Series[0].DataPoints.Add(i);
}

The following article describes how to use a Custom DashStyle: How to: Draw a Custom Dashed Line