Easiest option would be to have two versions of the graph and display the
appropriate one.

You can pretty much manipulate anything via the Graph object in code... if
you have to.

Changing the axis from Week to Month really means changing the chart's
source data and then changing the axis label.

eg. to change X axis label

Sample call: SetChartXTitle Me![MyGraph], "Monthly"

Function SetChartXTitle(ByRef pGraphObject As Object, pChartPeriod As
String)
 DoCmd.Hourglass True
 Dim MyGraph As Object
 Set MyGraph = pGraphObject.Object.Application
On Error Resume Next
 If pChartPeriod = "Monthly" Then
   'MyGraph.Chart.Axes(1, 1).TickLabels.Orientation =
xlTickLabelOrientationHorizontal
   MyGraph.Chart.Axes(1, 1).HasTitle = True
   MyGraph.Chart.Axes(1, 1).AxisTitle.Text = "Month"
 Else
  'MyGraph.Chart.Axes(1, 1).TickLabels.Orientation =
xlTickLabelOrientationUpward
   MyGraph.Chart.Axes(1, 1).HasTitle = True
   MyGraph.Chart.Axes(1, 1).AxisTitle.Text = "Week Commencing"
 End If
 MyGraph.Update
 Set MyGraph = Nothing
 DoCmd.Hourglass False
End Function

Sample code to reset the source data contained in the graph object (only
needed if you want to manually change the Graph's datasheet)

'Clear datasheet
 MyGraph.Datasheet.Cells.ClearContents

 'If no chart data then quit
 If rs.RecordCount = 0 Then Exit Function
 FieldCount = rs.Fields.Count
 If FieldCount < 2 Then Exit Function

 'Add data
 row = 2
 'rs.MoveFirst
 Do Until rs.EOF
   For col = 1 To FieldCount
     If IsNull(rs.Fields(col - 1)) Then
       MyGraph.Datasheet.Cells(row, col).ClearContents
     Else
       MyData = rs.Fields(col - 1)
       MyGraph.Datasheet.Cells(row, col) = MyData
     End If
   Next
   rs.MoveNext
   row = row + 1
 Loop