Use LineSeries.Points.RemoveAt(index)
Example:
(DataPlot.Series[0] as LineSeries).Points.Add(new DataPoint(xValue, yValue0));
(DataPlot.Series[1] as LineSeries).Points.Add(new DataPoint(xValue, yValue1));
if (valueRange > 10000) //points will accumulate until the x-axis reaches 10000
{ //after 10000
(DataPlot.Series[0] as LineSeries).Points.RemoveAt(0); //removes first point of first series
(DataPlot.Series[1] as LineSeries).Points.RemoveAt(0); //removes first point of second series
}
But you must use it together - adding one new point and removing one. Then the points will not accumulate and you will have x-axis of range you wish.