Currently the available appearance options of the line in Area, Line and ScatterLine Series are width and background color. Expose additional option for changing the dash type(e.g. dash, dashDot, Dot, etc). For the time being the following workaround can be used:
		<script>
			function pageLoad() {
				var chart = $find("<%=RadHtmlChart1.ClientID%>");
				//DashType: dash, dashDot, dot, longDash, longDashDot, longDashDotDot, solid
				chart._chartObject.options.series[0].dashType = "dot";
				chart.repaint();
			}
		</script>
		<telerik:RadHtmlChart ID="RadHtmlChart1" runat="server" Width="600px" Height="400px">
			<PlotArea>
				<Series>
					<telerik:LineSeries Name="Series 1">
						<SeriesItems>
							<telerik:CategorySeriesItem Y="30" />
							<telerik:CategorySeriesItem Y="10" />
							<telerik:CategorySeriesItem Y="20" />
						</SeriesItems>
					</telerik:LineSeries>
				</Series>
				<XAxis>
					<Items>
						<telerik:AxisItem LabelText="item 1" />
						<telerik:AxisItem LabelText="item 2" />
						<telerik:AxisItem LabelText="item 3" />
					</Items>
				</XAxis>
			</PlotArea>
		</telerik:RadHtmlChart>