This calls for an event based procedure. The event to use is the Chart_Calculate event. I presume your chart is not an embedded chart - it is an independent chart sheet.
Do the following:
Open the workbook
Go to the Visual Basic Editor (Alt+F11)
In the Project Explorer window, activate the module for the chart sheet by dbl-clicking its icon.
Copy the following code to this module:
Private Sub Chart_Calculate()
Dim Ser As Series
YRoundedTo = 10 'Change this to the multiple (5, 25, whatever) you wish to round down to
YMin = 10 ^ 9
For Each Ser In Me.SeriesCollection
YMin1 = WorksheetFunction.Min(Ser.Values)
If YMin1 < YMin Then YMin = YMin1
Next
YMin = WorksheetFunction.Floor(YMin, YRoundedTo)
Me.Axes(xlValue).MinimumScale = YMin
End Sub
The Y-axis will scale correctly every time the chart recalculates.
If you wish to refer to a cell on the worksheet to set min of y-axis, use the following code:
(I presume the cell is named "YMin")
Private Sub Chart_Calculate()
Me.Axes(xlValue).MinimumScale = Range("YMin")
End Sub
Hope you find this of help...