MarkerType.Circle problem

Hi, thank you very much for this amazing library!!
I have a problem and can't find a solution:
this is the result i'm tring to reach : 

the problem is:
if i set the markerType as circle i can't see any marker:



if i set the markerType as any other possibilities it work but adding to my chart a black rectangle around the chart:



This is my PlotModel:

public class OxyPlotModel

{

/// <summary>

/// Gets or sets the plot model that is shown in the demo apps.

/// </summary>

/// <value>My model.</value>

public PlotModel MyModel { get; set; }



/// <summary>

/// Initializes a new instance of the <see cref="OxyPlotSample.MyClass"/> class.

/// </summary>

public OxyPlotModel ()

{

var plotModel = new PlotModel {LegendSymbolLength = 30, PlotType = PlotType.Cartesian};


var xaxis = new DateTimeAxis{

Position = AxisPosition.Bottom,

TickStyle = TickStyle.None,

AxislineStyle = LineStyle.Solid,

AxislineColor = OxyPlot.OxyColor.FromRgb(153,153,153),

StringFormat = (CultureInfo.CurrentCulture.DateTimeFormat.GetAbbreviatedMonthName(1)) + "d HH",

IntervalType = DateTimeIntervalType.Hours



//FirstDateTime = new System.DateTime(2014,10,1)



//new System.DateTime(2014,10,1),

//new System.DateTime(2014,10,10),

//DateTimeIntervalType.Auto

};



var yaxis = new LinearAxis {

Position = AxisPosition.Left,

Minimum = 0.001f,

Maximum = 3,

MajorGridlineStyle = LineStyle.Solid,

TickStyle = TickStyle.None,

IntervalLength = 50

};



plotModel.Axes.Add (xaxis);

plotModel.Axes.Add (yaxis);





var series1 = new LineSeries {

Color = OxyPlot.OxyColor.FromRgb(44,169,173),

StrokeThickness = 1,

MarkerType = MarkerType.Circle,

MarkerStroke = OxyColors.Blue,

MarkerFill = OxyColors.SkyBlue,

//MarkerStrokeThickness = 5,

MarkerSize = 2,

DataFieldX="Date",

DataFieldY="Value",

TrackerFormatString="Date: {2:d HH}&#x0a;Value: {4}"

};



series1.Points.Add (new DataPoint (0.1, 0.7));

series1.Points.Add (new DataPoint (0.6, 0.9));

series1.Points.Add (new DataPoint (1.0, 0.85));

series1.Points.Add (new DataPoint (1.4, 0.95));

series1.Points.Add (new DataPoint (1.8, 1.2));

series1.Points.Add (new DataPoint (2.2, 1.7));

series1.Points.Add (new DataPoint (2.6, 1.7));

series1.Points.Add (new DataPoint (3.0, 0.7));



plotModel.Series.Add (series1);



this.MyModel = plotModel;

}




I can't understand why this happens and how get around it.
Do somebody know what the problem could be?

Thank you! (sorry for my terrible english^^)
Gabriele