You would just create a new x-axis object, and set its y-axis intercept so that it appears where you want, at the y-intercept of 10 and 20 in the example below.
TimeAxis xAxis2 = new TimeAxis(pTransform1);
xAxis2.SetAxisIntercept(10);
xAxis2.SetColor(Color.White);
chartVu.AddChartObject(xAxis2);
TimeAxis xAxis3 = new TimeAxis(pTransform1);
xAxis3.SetAxisIntercept(20);
xAxis3.SetColor(Color.White);
chartVu.AddChartObject(xAxis3);