Chart update working when macro executed step by step but not when at full execution

Dear All,

I've got a weird behavior of my macro. The data is x,y,z set of points (scan of a surface); I want to analyze the surface state, so at Y fixed I draw a plot (x,z), i get a x,y scatter plot of the section.
I need to detrend it so i use a 6th degree interpolation. I extract the interpolation from the trendline (TrendLineValue function in VBA) of the grape since LINEST at high degree is not working properly (I know it is dirty but I already spend a lot of time on that).
Then detrend the data, compute root mean square and store it in an array.
And I iterate through different Y (I call them steps).
The problem is between steps, the chart is not updated so trendline equation is not updated so value extracted by TrendLineValue are wrong....
I played around with DoEvents, Wait, application. Calculate but nothing is working....

Please do not mind all code related to FFT, I'm just trying to make this coefficient extraction working.
Thank you for your help.

Here is the file:
gofile.io/?c=q9X9YE

here is the code

Sub scanPlate()

Dim Step_start As Integer
Dim Step_stop As Integer
Dim max_peaks As New Collection
Dim RMS As New Collection

Application.ScreenUpdating = True
'dy
Step_start = ActiveSheet.Range("AH5").Value
Step_stop = ActiveSheet.Range("AI5").Value
'dx
ActiveSheet.Range("M11").Value = ActiveSheet.Range("AH4").Value
ActiveSheet.Range("M12").Value = ActiveSheet.Range("AI4").Value


Range("M1").Value = Step_start
'extract trend line coefficients of first step

TrendLineValue
Application.Calculate

'iteration through differetn steps of interest

Dim J As Integer

For J = Step_start To Step_stop Step 2

        'step no
        Range("M1").Value = J
     
        'FFT_MACRO: other macro that will be implemented later
        
        'extract trend line coefficients of step J
        TrendLineValue
               
        Debug.Print Range("M32").Value
        
        'store RMS and FTT value
        
        max_peaks.Add Range("M26").Value
        RMS.Add Range("M32").Value
        
        
        Debug.Print "step no: " & J
        
       
Next J

Debug.Print "scan finish"


'transform collection to arrays

arrPeaks = toArray(max_peaks)
arrRMS = toArray(RMS)

'display in the sheet all RMS and PEAK value

Range("AA1:AA119") = Application.Transpose(arrPeaks)
Range("AB1:AB119") = Application.Transpose(arrRMS)

Range("Z1").Value = Step_start


Application.Calculation = xlAutomatic

End Sub

Function toArray(col As Collection)
  Dim Arr() As Variant
  ReDim Arr(0 To col.Count - 1) As Variant
  
    Dim i As Integer
  For i = 1 To col.Count
      Arr(i - 1) = col(i)
  Next
  toArray = Arr
End Function


 
Function TrendLineValue()

    Dim c As Chart
    Dim t As Trendline
    Dim s As String
    Dim i As Integer
    Dim remaining As String
    Dim c0, c1, c2, c3, c4, c5, c6 As String
    Dim Arr As Variant
    ReDim Arr(0 To 7) As Variant
    
 
    
    Application.CalculateFull
    ' Get the trend line object
    ' this code assumes the first chart on the active sheet,
    '   and the first series, first trendline
    Set c = ActiveSheet.ChartObjects(2).Chart
    Set t = c.SeriesCollection(1).Trendlines(1)

    ' make sure equation is displayed
    t.DisplayRSquared = False
    t.DisplayEquation = True

    ' set number format to ensure accuracy
    '   adjust to suit requirements
    t.DataLabel.NumberFormat = "0.0000000000E+00"

    ' get the equation
    s = t.DataLabel.Text


    ' massage the equation string into form that will evaluate
    ' this code assumes 6rd order polynomial
    s = Replace(s, "y = ", "")
    s = Replace(s, " ", "")
    
    'extract each coefficients from string equation
    
     c6 = Left(s, InStr(1, s, "x") - 1)
    remaining = Right(s, Len(s) - InStr(1, s, "x") - 1)
    Arr(0) = c6
    
    c5 = Left(remaining, InStr(1, remaining, "x") - 1)
    remaining = Right(remaining, Len(remaining) - InStr(1, remaining, "x") - 1)
    Arr(1) = c5
    
     c4 = Left(remaining, InStr(1, remaining, "x") - 1)
    remaining = Right(remaining, Len(remaining) - InStr(1, remaining, "x") - 1)
    Arr(2) = c4
    
     c3 = Left(remaining, InStr(1, remaining, "x") - 1)
    remaining = Right(remaining, Len(remaining) - InStr(1, remaining, "x") - 1)
    Arr(3) = c3
    
     c2 = Left(remaining, InStr(1, remaining, "x") - 1)
    remaining = Right(remaining, Len(remaining) - InStr(1, remaining, "x") - 1)
    Arr(4) = c2
    
     c1 = Left(remaining, InStr(1, remaining, "x") - 1)
     remaining = Right(remaining, Len(remaining) - InStr(1, remaining, "x") - 0)
    Arr(5) = c1
    
    c0 = remaining
    Arr(6) = c0
    'display coefficeint on the sheets
    
Dim Destination As Range
Set Destination = ActiveSheet.Range("AU6")
Set Destination = Destination.Resize(UBound(Arr), 1)
Destination.Value = Application.Transpose(Arr)

End Function