property for each ChartSeriesItem in order to force it to stay at a certain position. In this case you can add multiple ChartSeriesItems with XValue=1 for John and another set for Martha. Here is an example:
RadChart1.AutoLayout = true;
RadChart1.SeriesOrientation = ChartSeriesOrientation.Horizontal;
ChartSeries ser = new ChartSeries("John");
ser.Type = ChartSeriesType.Gantt;
ser.Appearance.LabelAppearance.Visible = false;
ChartSeriesItem item = new ChartSeriesItem() { XValue = 1, YValue = new DateTime(2011, 12, 07, 11, 0, 0).ToOADate(), YValue2 = new DateTime(2011, 12, 07, 12, 0, 0).ToOADate() };
ser.Items.Add(item);
item = new ChartSeriesItem() { XValue = 1, YValue = new DateTime(2011, 12, 07, 14, 0, 0).ToOADate(), YValue2 = new DateTime(2011, 12, 07, 13, 0, 0).ToOADate() };
ser.Items.Add(item);
item = new ChartSeriesItem() { XValue = 1, YValue = new DateTime(2011, 12, 07, 16, 0, 0).ToOADate(), YValue2 = new DateTime(2011, 12, 07, 18, 0, 0).ToOADate() };
ser.Items.Add(item);
RadChart1.Series.Add(ser);
ser = new ChartSeries("Martha");
ser.Type = ChartSeriesType.Gantt;
ser.Appearance.LabelAppearance.Visible = false;
item = new ChartSeriesItem() { XValue = 2, YValue = new DateTime(2011, 12, 07, 5, 0, 0).ToOADate(), YValue2 = new DateTime(2011, 12, 07, 9, 0, 0).ToOADate() };
ser.Items.Add(item);
item = new ChartSeriesItem() { XValue = 2, YValue = new DateTime(2011, 12, 07, 11, 0, 0).ToOADate(), YValue2 = new DateTime(2011, 12, 07, 12, 0, 0).ToOADate() };
ser.Items.Add(item);
item = new ChartSeriesItem() { XValue = 2, YValue = new DateTime(2011, 12, 07, 14, 0, 0).ToOADate(), YValue2 = new DateTime(2011, 12, 07, 18, 0, 0).ToOADate() };
ser.Items.Add(item);
RadChart1.Series.Add(ser);
RadChart1.PlotArea.XAxis.AutoScale = false;
RadChart1.PlotArea.XAxis.AddRange(1, 2, 1);
RadChart1.PlotArea.XAxis.Items[0].TextBlock.Text = "John";
RadChart1.PlotArea.XAxis.Items[1].TextBlock.Text = "Martha";
RadChart1.PlotArea.YAxis.Appearance.ValueFormat = Telerik.Charting.Styles.ChartValueFormat.ShortTime;
RadChart1.PlotArea.YAxis.AutoScale = false;
RadChart1.PlotArea.YAxis.AddRange(new DateTime(2011, 12, 07).ToOADate(), new DateTime(2011, 12, 08).ToOADate(), 0.25);