public class Plotter
{
    private PlotModel model;
    private DateTime startDate = DateTime.Now.AddDays(-10);
    private DateTime endDate = DateTime.Now;

    private readonly List<OxyColor> colors = new List<OxyColor>
                                        {
                                            OxyColors.Green,
                                            OxyColors.IndianRed,
                                            OxyColors.Coral,
                                            OxyColors.Chartreuse,
                                            OxyColors.Azure
                                        };

    private readonly List<MarkerType> markerTypes = new List<MarkerType>
                                               {
                                                   MarkerType.Plus,
                                                   MarkerType.Star,
                                                   MarkerType.Diamond,
                                                   MarkerType.Triangle,
                                                   MarkerType.Cross
                                               };



    public Plotter()
    {
        model = new PlotModel { Title = "DateTimeAxis" };
    }


    public PlotModel PrintChart()
    {
        var minValue = DateTimeAxis.ToDouble(startDate);
        var maxValue = DateTimeAxis.ToDouble(endDate);
        model.Axes.Add(new DateTimeAxis { Position = AxisPosition.Bottom, Minimum = minValue, Maximum = maxValue, StringFormat = "HH:mm" });
        return model;
    }

public void PrintChart()
    {
        Plotter pt = new Plotter();
        frGraph.Model = pt.PrintChart();
    }

private void startDrawing_Click(object sender, EventArgs e)
    {
       PrintChart();
    }

private void InitializeTimerForChart()
{
    tickCount.Interval = References.TIMER_INTERVAL; // (1000 мс)
    this.tickCount.Tick += new System.EventHandler(this.tickCount_Tick);
    tickCount.Enabled = true;
}
private void tickCount_Tick(object sender, EventArgs e)
{
    if (InvokeRequired)
    {
        Invoke((MethodInvoker)delegate ()
        {
            frGraph.Invalidate();//PrintChart();
        });
    }
}