WPF vs WinForms Oxyplot Customize Tooltip/Tracker

Looking into the source code of the component, it's not really a tooltip, it's a Label. An option could be getting the private trackingLabel and apply changes on the region of the control (or ideally replace it with a custom label which supports that shape).

Also to draw the crosshair tracker, you can handle Paint event of the plot, and draw the horizontal and vertical line based on the center of label.

well, what I mentioned above is just a quick workaround which results in the following:

Using the following code:

private Label trackerLabel;
private void Form1_Load(object sender, EventArgs e)
{
    var trackerLabelField = plotView1.GetType().GetField("trackerLabel",
        System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Instance);
    trackerLabel = new BalloonLabel()
    {
        Visible = false,
        Parent = plotView1
    };
    trackerLabelField.SetValue(plotView1, trackerLabel);

    var myModel = new PlotModel { Title = "Example 1" };
    myModel.Series.Add(new FunctionSeries(Math.Cos, 0, 10, 0.1, "cos(x)"));
    this.plotView1.Model = myModel;
    this.plotView1.Paint += PlotView1_Paint;
    trackerLabel.VisibleChanged += TrackerLabel_VisibleChanged;
}
private void TrackerLabel_VisibleChanged(object sender, EventArgs e)
{
    plotView1.Invalidate();
}
private void PlotView1_Paint(object sender, PaintEventArgs e)
{
    if (trackerLabel.Visible)
    {
        var r = plotView1.Model.PlotArea;
        e.Graphics.DrawLine(Pens.Blue, trackerLabel.Left + trackerLabel.Width / 2, (int)r.Top,
            trackerLabel.Left + trackerLabel.Width / 2, (int)r.Bottom);
        e.Graphics.DrawLine(Pens.Blue, (int)r.Left, trackerLabel.Bottom, (int)r.Right, trackerLabel.Bottom);
    }
}

And this balloon label, which I assume is a good-enough start point:

using System;
using System.Drawing;
using System.Drawing.Drawing2D;
using System.Windows.Forms;
public class BalloonLabel : Label
{
    public BalloonLabel()
    {
        BackColor = SystemColors.Info;
        Padding = new Padding(5, 5, 5, 20);
        ArrowSize = new Size(10, 20);
        AutoSize = true;
    }
    private Size arrowSize;
    public Size ArrowSize
    {
        get { return arrowSize; }
        set
        {
            arrowSize = value;
            this.RecreateRegion();
        }
    }

    private GraphicsPath GetBalloon(Rectangle bounds)
    {
        GraphicsPath path = new GraphicsPath();
        path.StartFigure();

        if (arrowSize.Width > 0 && arrowSize.Height > 0)
        {
            path.AddLine(bounds.Left, bounds.Bottom - arrowSize.Height / 2, bounds.Left, bounds.Top);
            path.AddLine(bounds.Left, bounds.Top, bounds.Right, bounds.Top);
            path.AddLine(bounds.Right, bounds.Top, bounds.Right, bounds.Bottom - arrowSize.Height / 2);
            path.AddLine(bounds.Right, bounds.Bottom - arrowSize.Height / 2, bounds.Left + bounds.Width / 2 + arrowSize.Width / 2, bounds.Bottom - arrowSize.Height / 2);
            path.AddLine(bounds.Left + bounds.Width / 2 + arrowSize.Width / 2, bounds.Bottom - arrowSize.Height / 2,
                bounds.Left + bounds.Width / 2, bounds.Bottom);
            path.AddLine(bounds.Left + bounds.Width / 2, bounds.Bottom,
                bounds.Left + bounds.Width / 2 - arrowSize.Width / 2, bounds.Bottom - arrowSize.Height / 2);
            path.AddLine(bounds.Left + bounds.Width / 2 - arrowSize.Width / 2, bounds.Bottom - arrowSize.Height / 2,
                bounds.Left, bounds.Bottom - arrowSize.Height / 2);
        }
        else
        {
            path.AddLine(bounds.Left, bounds.Bottom, bounds.Left, bounds.Top);
            path.AddLine(bounds.Left, bounds.Top, bounds.Right, bounds.Top);
            path.AddLine(bounds.Right, bounds.Top, bounds.Right, bounds.Bottom);
            path.AddLine(bounds.Right, bounds.Bottom, bounds.Left, bounds.Bottom);
        }
        path.CloseFigure();
        return path;
    }

    private void RecreateRegion()
    {
        var r = ClientRectangle;
        using (var path = GetBalloon(r))
            this.Region = new Region(path);
        this.Invalidate();
    }
    protected override void OnSizeChanged(EventArgs e)
    {
        base.OnSizeChanged(e);
        this.RecreateRegion();
    }
    protected override void OnPaint(PaintEventArgs e)
    {
        base.OnPaint(e);
        var r = ClientRectangle;
        r.Inflate(-1, -1);
        using (var path = GetBalloon(r))
        {
            e.Graphics.SmoothingMode = SmoothingMode.AntiAlias;
            using (var pen = new Pen(Color.Gray, 1) { Alignment = PenAlignment.Inset })
                e.Graphics.DrawPath(pen, path);
            if (Parent != null)
                Parent.Invalidate();
        }
    }
}

And here is the animated result: