主题：如何在小计列中计算出库存的累计数是31呢?

Dim t As Table = Tables("例子二")
Dim g As Subtotalgroup
t.SubtotalGroups.Clear()
t.GroupAboveData = False
t.TreeVisible = False
t.SpillNode = True

g = New Subtotalgroup
g.Aggregate = AggregateEnum.Sum
g.GroupOn = "日期"
g.TotalOn = "入库,出库,库存"
g.Caption = "{0}月 小计"
g.DateGroup = DateGroupEnum.Month
t.SubtotalGroups.Add(g)

g = New Subtotalgroup
g.Aggregate = AggregateEnum.Sum
g.GroupOn = "日期"
g.TotalOn = "入库,出库,库存"
g.Caption = "{0}月 累计"
g.DateGroup = DateGroupEnum.Month
t.SubtotalGroups.Add(g)
t.Subtotal

For i As Integer = 0 To CurrentTable.Rows.Count(True) - 1
    Dim r As Row = CurrentTable.Rows(i,True)
    If r.IsGroup
        r("库存") = r("入库")- r("出库")
    End If
Next