Sub test(wb as Workbook)
 
Dim Yneeded As Double, Xfound As Double
Yneeded = 2
 
Dim ch As Chart
Set ch = wb.Charts(1) ' Тебе это, скорее всего, не нужно

Dim s As Series
Set s = ch.SeriesCollection(1)
 
Dim vX As Variant, vY As Variant
vX = s.XValues
vY = s.Values
 
Dim i As Long
For i = 1 To UBound(vX)
If vY(i) = Yneeded Then
Xfound = vX(i)
Exit For
End If
Next i
 
End Sub