How to dynamically do a linear interpolation of da

Question

I have this data

Date    Data
8/25/2017   980
8/24/2017   64
8/23/2017   593   
8/22/2017   595
8/21/2017   
8/20/2017   
8/19/2017   794
8/18/2017   437
8/17/2017   
8/16/2017   
8/15/2017   
8/14/2017   629

What if i wanted (794-595)/3 in cell 21st August and 2*(794-595)/3 in cell 22nd August and similarly (629-437)/4 in 17th August, 2*(629-437)/4 in 16th August etc...

How to do this dynamically without regard to the number of missing values in between

Answer 1

The code below provides one solution, and the animated gif shows the code solving your problem. Of course, there are many ways of solving this problem, but this is simple. At least this shows you what inputs will be needed. It assumes the x-values increment by 1 (as you have in your data). Otherwise you'd need to use something like the slope & intercept functions.

Option Explicit
Sub interpolate()
Dim r As Range, cell As Range
Set r = Application.InputBox("select interpolation range", , , Type:=8)
For Each cell In r
  If cell = "" Then
    cell = r(1) + (r(r.Rows.Count) - r(1)) * (cell.row - r.row) / (r(r.Rows.Count).row - r(1).row)
  End If
Next
End Sub