Interpolation/extrapolation using ActiveX TeeChart component

[Ritchie]

Does anyone know if there is a way to find the y coordinate for a given x coordinate for an XY series when there isn't a matching x coordinate in the series?

eg If a series contains points (2, 10), (4, 30), (9, 42.56), (10.2, 24.1)...
... is there a way to get the y coordinate when x=7 without having to perform my own interpolation?

I know that it seems as if I'm being lazy, but there is a performance issue when I calculate it myself and I am hoping there is a more efficient built-in method to interpolate (and/or extrapolate).

Thanks in advance,
Richard

[Narcís]

Hi Richard,

You can do it using a vertical cursor tool and implementing it's OnChange event:

Code: Select all

Private Sub Form_Load()
    TChart1.Series(0).FillSampleValues 10
    
    TChart1.Tools.Items(0).asTeeCursor.Style = cssVertical
End Sub

Private Sub TChart1_OnCursorToolChange(ByVal Tool As Long, ByVal X As Long, ByVal Y As Long, ByVal XVal As Double, ByVal YVal As Double, ByVal Series As Long, ByVal ValueIndex As Long)
    Dim ValuesStr As String

    ValuesStr = "X: " + CStr(TChart1.Tools.Items(0).asTeeCursor.XVal) _
            + ", Y: " + CStr(TChart1.Tools.Items(0).asTeeCursor.YVal)

    TChart1.Header.Text.Clear
    TChart1.Header.Text.Add (ValuesStr)
End Sub

[Ritchie]

Hi Narcis,

I tried your code and found that moving the vertical cursor only changes the x axis value - the y axis value never changes. (I also notice that if both horizontal and vertical are enabled, changing the y value doesn't report the correspondig x value either.)

In my program I want to programmatically iterate through each x value in one series and find what the corresponding y value on another series is for the same x value.

[Marjan]

... is there a way to get the y coordinate when x=7 without having to perform my own interpolation?

I think not. You'll have to calculate local linear approximate and then use it for calculating y=y(x). It's not that complication + it really doesn't take heavy tool on cpu <gg>. I did an example for Delphi, but the same approach can be used for AX or .NET version. A discusion can be found
here.

[Ritchie]

The reason I was hoping to avoid doing the linear interpolation is that I have sporradic data going back to 1976 and finding the closest point prior to the intersection could be quite time consuming. However, where needs must.....

Thank you all for the help.
Ritchie

[Marjan]

Hi.

have sporradic data going back to 1976

Ok, the number of points can slow down the search for closest points. But there is no need to search all points - you can do a left/right search once you have initial reference point (since x values are already sorted).

[Ritchie]

Just thinking....My data source is via ADO and I can run a quick query to find the 1st date in my second curve (ie the one I want to get the y value from) which comes after the seek date, and then use a vertical cursor attached to the 2nd curve to get the index and y2 value, and hence the date at the previous index and the y1 value - Problem solved !

Thanks again
Ritchie