Dim dXMin as Double, dXMax as Double
Dim vXVals as Variant
vXVals = ActiveChart.SeriesCollection(1).XValues
dXMin = WorksheetFunction.Min(vXVals)
dXMax = WorksheetFunction.Max(vXVals)
With ActiveChart.Axes(xlCategory)
.MinimumScale = dxMin
.MaximumScale = dXMax
End With