How to Select a Specific Bar on a 2D Graph with the Mouse

MORE INFORMATION

The graph control does not have a built-in method that you can use to discover which bar or slice of a 2D Bar Graph your mouse pointer is currently over. However, the graph control's ColorData property does correspond to Visual Basic's QBColor() function. Therefore, if you display the Graph in a picture box by assigning the Graph's picture property to a picture box, you can use the picture box's Point(x,y) method to compare the color of the bar at the position of your mouse. You can use this method to differentiate up to 14 different points.

Step-by-Step Example

1. Start a new project in Visual Basic (ALT, F, N). Form1 is created by default.
2. Add a Label and Graph control. Then add a Picture control on top of the graph control; give both controls identical Top, Left, Width, and Height property values. Set the GraphType to Pie or Bar chart.
3. Add the following code to the Form_Load event:

      Sub Form_Load ()
         Picture1.Move Graph1.Left, Graph1.Top, Graph1.Width, Graph1.Height
         Graph1.Visible = False
   
         ' Set the color to between 1 and the number of slices or Bars you
         ' will have (up to 14).
         For I = 1 to Graph1.NumPoints
             Graph1.ThisPoint = I
             Graph1.ColorData = I
         Next I
         Graph1.DrawMode = 2                'Draw the graph
         Picture1.Picture = Graph1.Picture  'Display in picture box
      End Sub
   						

4. Add the following code to the Picture1_MouseDown event:

      ' Place the following two lines on one, single line:
      Sub Picture1_MouseDown (Button As Integer, Shift As Integer,
         X As Single, Y As Single)
   
         Select Case Picture1.Point(X, Y)
            Case QBColor(1)
                label1.Caption = "1st"
            Case QBColor(2)
                label1.Caption = "2nd"
            Case QBColor(3)
                label1.Caption = "3rd"
            Case QBColor(4)
                label1.Caption = "4th"
            Case QBColor(5)
                label1.Caption = "5th"
            Case QBColor(6)
                label1.Caption = "6th"
            Case QBColor(7)
                label1.Caption = "7th"
            Case QBColor(8)
                label1.Caption = "8th"
            Case QBColor(9)
                label1.Caption = "9th"
            Case QBColor(10)
                label1.Caption = "10th"
            Case QBColor(11)
                label1.Caption = "11th"
            Case QBColor(12)
                label1.Caption = "12th"
            Case QBColor(13)
                label1.Caption = "13th"
            Case QBColor(14)
                label1.Caption = "14th"
         End Select
      End Sub
   						

5. Save the project (ALT, F, S).
6. Run the program by pressing the F5 key. When you click any of the bars or Slices, the label will tell you which one you selected.

Notes

• To change the Data in the graph control and refresh the picture use:

     Graph1.DrawMode = 2
     Picture1.Picture = Graph1.Picture
  						

• ColorData 0 is Black and 15 is White. By default the background is white and the lines are black, so don't use these colors. If you do, you might get invalid information.
• This method will not work reliably if multiple datasets (NumSets > 1) or 3D versions of the bar graph are used. For multiple datasets, you may choose a specific color for each set. Then differentiate which set the user selects. A 3D bar uses black pixels for the shading effect, so it does not lend itself to this technique.