WPF Chart control: Line series

I have a data entry setean every two seconds, these data graphing on a LineChart want and I want each entry is a different point LineChart not this replaces the data at the same point, I have this:

DateTime.Now.ToOADatevar converted =();  DatosvariablePoint=newPoint();  datosvariable.X = converted;  datosvariable.Y = item.UltimaMedicion.Value;  this.chartline.Points.Add(datosvariable);

But when I compile I get the following error: Object reference not set to an instance of an object

What do I have wrong or what is the correct way?