Hi Oystein,
I will try to do that when I get time. I have my boss breathing down my neck for a slipping deadline but I will do my best! :) Thanks for all your hard work, cheers dude.
Ryan

Commenting out that whole block seems to make the problem go away. I might be being thick, but it seems like 'result' isn't actually being used for anything?! 

Ok, I have tracked it down to the 'smooth' option on the series. Switching smoothing OFF eliminates the problem. The line that triggers the null ref in LineSeries.GetNearestPoint()  is in this block, when it tries to access the result's item:

if (interpolate && this.Smooth && this.SmoothedPoints != null)
{
var result = this.GetNearestInterpolatedPointInternal(this.SmoothedPoints, point);
result.Text = this.Format( this.TrackerFormatString, 
result.Item, // Crashes here!!
this.Title, 
this.XAxis.Title ?? XYAxisSeries.DefaultXAxisTitle,
this.XAxis.GetValue(result.DataPoint.X),
this.YAxis.Title ?? XYAxisSeries.DefaultYAxisTitle
this.YAxis.GetValue(result.DataPoint.Y));
}

At a guess, when the 'normal' point collection is modified, the smoothed points aren't updated?? I am using the Xamarin component so I had to just make my 'own' class which is a duplicate of LineSeries and step through it, but that means I cant dive any deeper very easily and I don't have time right now.

Cheers,
Ryan

Here's the trace:

System.NullReferenceException: Object reference not set to an instance of an object
at OxyPlot.Series.LineSeries.GetNearestPoint (ScreenPoint point, Boolean interpolate) [0x00000] in <filename unknown>:0
at OxyPlot.Series.Series.HitTestOverride (OxyPlot.HitTestArguments args) [0x00000] in <filename unknown>:0
at OxyPlot.UIElement.HitTest (OxyPlot.HitTestArguments args) [0x00000] in <filename unknown>:0
at OxyPlot.Model+<HitTest>d__0.MoveNext () [0x00000] in <filename unknown>:0
at OxyPlot.Model.HandleTouchStarted (System.Object sender, OxyPlot.OxyTouchEventArgs e) [0x00000] in <filename unknown>:0
at OxyPlot.ControllerBase.HandleTouchStarted (IView view, OxyPlot.OxyTouchEventArgs args) [0x00000] in <filename unknown>:0
at OxyPlot.Xamarin.iOS.PlotView.TouchesBegan (Foundation.NSSet touches, UIKit.UIEvent evt) [0x00000] in <filename unknown>:0
at at (wrapper managed-to-native) UIKit.UIApplication:UIApplicationMain (int,string[],intptr,intptr)
at UIKit.UIApplication.Main (System.String[] args, IntPtr principal, IntPtr delegate) [0x00005] in /Developer/MonoTouch/Source/monotouch/src/UIKit/UIApplication.cs:62
at UIKit.UIApplication.Main (System.String[] args, System.String principalClassName, System.String delegateClassName) [0x00038] in /Developer/MonoTouch/Source/monotouch/src/UIKit/UIApplication.cs:46
at Verto.iOS.Application.Main (System.String[] args) [0x00008] in /Users/ryanpalmer/Documents/Clients/Git_Repos/V/Verto/verto iOS/Verto.iOS/Main.cs:17

I'm using the Android version in Xamarin at the moment and it's working great. Not found any issues yet.