Get all DataPoints within a 'rubber-band' rectangle in a Chart control

Question

I have an X-Y plot in a .NET 4.0 WinForms chart control. I am trying to implement rubber-band selection, so that the user could click and drag the mouse to create a rectangle on the plot, thus selecting all the points that lie within this rectangle.

While I was able to code up the drawing of the rectangle, I am now trying to identify the Datapoints that lie within this rectangle. Here is the relevant code:

public partial class Form1 : Form
{
    System.Drawing.Point _fromPosition;
    Rectangle _selectionRectangle;

    public Form1()
    {
        InitializeComponent();            
    }

    private void chart1_MouseMove(object sender, MouseEventArgs e)
    {
        // As the mouse moves, update the dimensions of the rectangle
        if (e.Button == MouseButtons.Left)
        {
            Point p = e.Location;
            int x = Math.Min(_fromPosition.X, p.X);
            int y = Math.Min(_fromPosition.Y, p.Y);
            int w = Math.Abs(p.X - _fromPosition.X);
            int h = Math.Abs(p.Y - _fromPosition.Y);
            _selectionRectangle = new Rectangle(x, y, w, h);

            // Reset Data Point Attributes
            foreach (DataPoint point in chart1.Series[0].Points)
            {
                point.BackSecondaryColor = Color.Black;
                point.BackHatchStyle = ChartHatchStyle.None;
                point.BorderWidth = 1;
            }   
            this.Invalidate();
        }                     
    }

    private void chart1_MouseDown(object sender, MouseEventArgs e)
    {
        // This is the starting position of the rectangle
        _fromPosition = e.Location;
    }

    private void chart1_Paint(object sender, PaintEventArgs e)
    {
        e.Graphics.DrawRectangle(new Pen(Color.Blue, 2), _selectionRectangle);
        foreach (DataPoint point in chart1.Series[0].Points)
        {                
            // Check if the data point lies within the rectangle 
            if (_selectionRectangle.Contains(???))))
            {
                // How do I convert DataPoint into Point?
            }
        }
    }
}

What I am trying to do is query each DataPoint in the series and check if it lies within the Rectangle. Here, I am unable to transform each DataPoint into its corresponding Point. It seems pretty straightforward, so I am either missing something basic here or approaching the problem incorrectly.

I should also add that I referred to similar questions here and here, but they do not talk about how to actually identify DataPoints within the rectangle.

Any direction would be appreciated!

Answer 1

I have shown how to cheat the Chart into helping to get at the coordinates of DataPoints in the Paint event here.

But as you want to pick them up in the Paint event anyway, no cheating is needed..:

I define a List to collect the lassoed DataPoints:

List<DataPoint> dataPoints = new List<DataPoint>();

And I clear it on each new selection:

void chart1_MouseDown(object sender, MouseEventArgs e)
{
    _fromPosition = e.Location;
    dataPoints.Clear();
}

At the end I can write out the results:

void chart1_MouseUp(object sender, MouseEventArgs e)
{
    foreach(DataPoint pt in dataPoints)
        Console.WriteLine("found:" + pt.ToString() +
            " at " + chart1.Series[0].Points.IndexOf(pt));
}

And in the Paint event we make use of the ValueToPixelPosition method of the two axes:

void chart1_Paint(object sender, PaintEventArgs e)
{
    using (Pen pen = new Pen(Color.Blue, 2) // dispose of my Pen
          {DashStyle = System.Drawing.Drawing2D.DashStyle.Dot})
       e.Graphics.DrawRectangle(pen, _selectionRectangle);
    foreach (DataPoint point in chart1.Series[0].Points)
    {   // !! officially these functions are only reliable in a paint event!!
        double x = chart1.ChartAreas[0].AxisX.ValueToPixelPosition(point.XValue);
        double y = chart1.ChartAreas[0].AxisY.ValueToPixelPosition(point.YValues[0]);
        PointF pt = new PointF((float)x,(float)y);

        // Check if the data point lies within the rectangle 
        if (_selectionRectangle.Contains(Point.Round(pt)))
        {
            if (!dataPoints.Contains(point)) dataPoints.Add(point);
        }
    }      
}