Well this is an interesting request. And of course VBA can easily deliver.


Option Explicit
 
Sub DrawLines()
Dim TopX            As Double
    Dim LeftX           As Long
    Dim Ln              As Shape
    Dim a               As Double
    Dim b               As Double
    Dim Cel1            As Range
    Dim Cel2            As Range
Set Cel1 = Range("A12")
    Set Cel2 = Range("F4")
TopX = Cel1.Top
    LeftX = Cel1.Left
    a = Abs(Cel2.Offset(0, 1).Left - Cel1.Left)
    b = Abs(Cel1.Top - Cel2.Offset(1, 0).Top)
    Set Ln = ActiveSheet.Shapes.AddLine(1, 1, 1, 1)
Ln.Width = a
    Ln.Height = b
    Ln.Top = TopX
    Ln.Left = LeftX
End Sub
You can use the Application.InputBox or a RefEdit Control to get the cell addresses.

Note that this works if the first cell is above and to the left of the second cell. To account for other possibilities we would need to add some more code.