After a lot of search, I managed to do this:

Sub Macro1()
minim = Range("param!i35").Value2
maxim = Range("param!i45").Value2
ActiveSheet.ChartObjects("Chart 7491").Activate
ActiveChart.Axes(xlCategory).Select
With ActiveChart.Axes(xlCategory)
.MinimumScale = minim
.MaximumScale = maxim
End With
ActiveChart.ChartArea.Select
ActiveWindow.Visible = False
End Sub

The workbook contains many graphs:
- 1 sheet with 1 scatter graph
- 1 sheet with 8 scatter graphs
- 4 sheets with 2 3D-graphs and 23 scatter graphs each
- a couple of sheets without graphs

There are three things I don't know how to do:
1) To assign the same minimum and maximum to the x-axes of
all the scatter graphs in one shot.
2) To change the names of the graphs ("Chart 7491" is awkward).
3) To unselect the graphs after the execution of the macro
(I have to press Escape, or else any movement of the cursor
would drag the graph).

Any help would be much appreciated.