To reproduce:

Setup the following RadChartView:

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


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


You will see that although that you have changed the BorderDashStyle of the Element, the LineSerie will be drawn normally.