(687;10)
(12;20)
(24;30)
(10125;40)
(0;50)
(111;60)
(-2;70)

private ZedGraph.PointPair findPoint(ZedGraph.CurveItem curve, double x)
{
    if (curve.NPts == 1)
        return curve.Points[0];

    // Look for the min and max value
    double min = curve.Points[0].X;
    double max = curve.Points[curve.NPts - 1].X;

    double delta = curve.Points[1].X - min;

    // Prevent error if mouse is out of bounds.
    if (x > max)
        x = curve.Points[curve.NPts - 1].X;
    else if (x < min)
        x = curve.Points[0].X;

    double index = (x - min) / delta;
    int nearestIndex = (int)Math.Round(index, 0);

    if (nearestIndex < curve.NPts)
    {
        return curve.Points[nearestIndex];
    }
    return curve.Points[curve.NPts - 1];
}

private ZedGraph.PointPair findPoint(ZedGraph.CurveItem curve, double x)
{
    // Look for the min and max value
    double min = curve.Points[0].X;
    double max = curve.Points[curve.NPts - 1].X;

    // Prevent error if mouse is out of bounds.
    if (x <= min) return curve.Points[0];
    if (x >= max) return curve.Points[curve.NPts - 1];

    double delta = (max - min) / (curve.NPts - 1);
    int index = (int)Math.round((x - min) / delta, 0);

    Debug.Assert(Math.Abs(x - curve.Points[index].X) <= delta / 2, "Interpolation error");
    return curve.Points[index];
}