public partial class Form1 : Form
{
    ScottPlot.Plottable.VLine VLine1;
    ScottPlot.Plottable.HLine HLine1;

    double[] YSnapPositions = DataGen.Consecutive(11, 0.2, -1);
    double[] XSnapPositions = DataGen.Consecutive(11, 5D);

    public Form1()
    {
        InitializeComponent();

        formsPlot1.Plot.AddSignal(DataGen.Sin(51), 1, Color.Black);
        formsPlot1.Plot.AddSignal(DataGen.Cos(51), 1, Color.Gray);

        VLine1 = formsPlot1.Plot.AddVerticalLine(23, Color.Blue);
        VLine1.DragEnabled = true;

        HLine1 = formsPlot1.Plot.AddHorizontalLine(.2, Color.Green);
        HLine1.DragEnabled = true;

        formsPlot1.PlottableDropped += FormsPlot1_PlottableDropped;

        formsPlot1.Refresh();
    }

    private void FormsPlot1_PlottableDropped(object sender, EventArgs e)
    {
        (double mouseX, double mouseY) = formsPlot1.GetMouseCoordinates();

        if (sender is ScottPlot.Plottable.VLine vline)
        {
            vline.X = GetClosestPosition(mouseX, XSnapPositions);
            formsPlot1.Render();
        }
        else if (sender is ScottPlot.Plottable.HLine hline)
        {
            hline.Y = GetClosestPosition(mouseY, YSnapPositions);
            formsPlot1.Render();
        }
    }

    private double GetClosestPosition(double mouse, double[] snaps)
    {
        double closestDistance = double.MaxValue;
        double closestPosition = double.MaxValue;

        for (int i = 0; i < snaps.Length; i++)
        {
            double distance = Math.Abs(mouse - snaps[i]);
            if (distance < closestDistance)
            {
                closestDistance = distance;
                closestPosition = snaps[i];
            }
        }

        return closestPosition;
    }
}