Accounts down the side, months across the top: that's how finance data arrives and how humans read it. It's also the one shape a pivot table, a lookup and a chart all refuse to work with properly. Unpivoting turns one row of twelve months into twelve rows of account-month-amount, which is what every tool actually wants. The loop is short — walk the columns inside a walk of the rows, writing one output row per cell — and it replaces a reshaping job people do by hand every single month.

Sub UnpivotToList()
    Dim src As Worksheet, out As Worksheet
    Dim lastRow As Long, lastCol As Long
    Dim r As Long, c As Long, n As Long

    Set src = ThisWorkbook.Worksheets("Grid")

    ' Rebuild the output sheet from scratch each run.
    On Error Resume Next
    Application.DisplayAlerts = False
    ThisWorkbook.Worksheets("List").Delete
    Application.DisplayAlerts = True
    On Error GoTo 0

    Set out = ThisWorkbook.Worksheets.Add
    out.Name = "List"
    out.Range("A1:C1").Value = Array("Account", "Month", "Amount")

    lastRow = src.Cells(src.Rows.Count, 1).End(xlUp).Row
    lastCol = src.Cells(1, src.Columns.Count).End(xlToLeft).Column

    n = 1
    For r = 2 To lastRow                    ' each account
        For c = 2 To lastCol                ' each month column
            If src.Cells(r, c).Value <> "" Then
                n = n + 1
                out.Cells(n, 1).Value = src.Cells(r, 1).Value   ' account
                out.Cells(n, 2).Value = src.Cells(1, c).Value   ' month header
                out.Cells(n, 3).Value = src.Cells(r, c).Value   ' amount
            End If
        Next c
    Next r

    out.Columns("A:C").AutoFit
    MsgBox "Wrote " & n - 1 & " rows.", vbInformation
End Sub

Paste into the Visual Basic Editor (Alt + F11 → Insert → Module), then run and save as macro-enabled (.xlsm).

• Make a monthly grid usable by a pivot table.
• Turn a budget laid out by column into something a lookup can read.
• Prepare data for a chart that wants one series per row.

A simulation — press Run to perform what the macro does to a sample workbook.

• Power Query — Unpivot Columns — Genuinely better if you'll do this repeatedly: select the month columns, right-click, Unpivot. No code, and it refreshes.
• A pivot table's own source — Doesn't help — a pivot needs the list shape to begin with, which is the whole problem.