I'm trying to use MS Chart with custom controls. My purpose is to:

• Highlight only a segment of the line that connects two neighboring points on mouse hover over that piece
• Find indexes of those two neighboring points (I need that for being able to drag that line by moving two points simultaneously)

Kind of illustration:

For now I can detect a hover over a line on the chart by using the approach described here. But I'm stuck in finding indexes or at least coordinates of those two points.

CodePudding user response：

So the original idea from this question was to find nearest points by x (assuming that all the series has x-values are indeed steadily increasing) and then calculate y-value. But I have a little improved that and added support for completely vertical lines. So here is my code for capturing the needed line:

 private static GrippedLine? LineHitTest(Series series, double xPos, double yPos, Axis xAxis, Axis yAxis)
    {
        double xPixelPos = xAxis.PixelPositionToValue(xPos);
        double yPixelPos = yAxis.PixelPositionToValue(yPos);
        DataPoint[] neighbors = new DataPoint[2];
        neighbors[0] = series.Points.Last(x => x.XValue <= xPixelPos);
        neighbors[1] = series.Points.First(x => x.XValue >= xPixelPos);

        DataPoint[] verticalMates;
        foreach (DataPoint neighbor in neighbors)
        {
            if (Math.Abs(neighbor.XValue - xPixelPos) < LINE_GRIP_REGION)
            {
                verticalMates = series.Points.FindAllByValue(neighbor.XValue, "X").ToArray();
                if (verticalMates.Length > 1)
                {
                    if (verticalMates.Length > 2)
                    {
                        if (verticalMates[0].YValues[0] < verticalMates[verticalMates.Length - 1].YValues[0])
                        {
                            neighbors[0] = verticalMates.LastOrDefault(y => y.YValues[0] < yPixelPos);
                            neighbors[1] = verticalMates.FirstOrDefault(y => y.YValues[0] >= yPixelPos);
                        }
                        else
                        {
                            neighbors[0] = verticalMates.LastOrDefault(y => y.YValues[0] > yPixelPos);
                            neighbors[1] = verticalMates.FirstOrDefault(y => y.YValues[0] <= yPixelPos);
                        }
                    }
                    else
                    {
                        neighbors[0] = verticalMates[0];
                        neighbors[1] = verticalMates[1];
                    }
                    break;
                }
            }
        }

        double x0 = xAxis.ValueToPixelPosition(neighbors[0].XValue);
        double y0 = yAxis.ValueToPixelPosition(neighbors[0].YValues[0]);

        double x1 = xAxis.ValueToPixelPosition(neighbors[1].XValue);
        double y1 = yAxis.ValueToPixelPosition(neighbors[1].YValues[0]);

        double Yinterpolated = y0   (y1 - y0) * (xPos - x0) / (x1 - x0);

        int[] linePoints = new int[2];
        // if mouse Y position is near the calculated OR the line is vertical
        if (Math.Abs(Yinterpolated - yPos) < LINE_GRIP_REGION || neighbors[0].XValue == neighbors[1].XValue)
        {
            linePoints[0] = series.Points.IndexOf(neighbors[0]);
            linePoints[1] = series.Points.IndexOf(neighbors[1]);

        }
        else
        {
            return null;
        }
        return new GrippedLine()
        {
            startLinePointIndex = linePoints[0],
            endLinePointIndex = linePoints[1],
            x0Correction = neighbors[0].XValue - xPixelPos,
            y0Correction = neighbors[0].YValues[0] - yPixelPos,
            x1Correction = neighbors[1].XValue - xPixelPos,
            y1Correction = neighbors[1].YValues[0] - yPixelPos
        };
    }