奥が深い．．．

グラフ作成のサンプルをWebで探すと，

ActiveSheet.ChartObjects.Add Left:=0, Top:=0, Width:=320, Height:=240
ActiveChart.ChartType = xlXYScatter
ActiveChart.ほげほげの設定
...

こんなのがでてきます．
でも，動きません．
ActiveChartを使うと"Error 91"がでてきます．

ウンウンうなりながら，試行錯誤を繰り返したところ，

' コードを標準モジュールへ移動させた後．．．

Worksheets(sheetname).Select

' グラフ作成
With ActiveSheet
    Dim rc As Range
    Set rc = .Range(Cells(2, 1 + codeCalcNum * stepCol + 1), Cells(22, 1 + codeCalcNum * stepCol + 1 + 7))
    
    Dim performChart As ChartObject
    'Set performChart = Worksheets(sheetname).ChartObjects.Add(Left:=rc.Left, Top:=rc.Top, Width:=rc.Width, Height:=rc.Height)
    Set performChart = .ChartObjects.Add(Left:=rc.Left, Top:=rc.Top, Width:=rc.Width, Height:=rc.Height)
    
    ' グラフ形式を指定し，データをセット
    performChart.chart.ChartType = xlLineMarkers
    'performChart.chart.SetSourceData Source:=.Range(Cells(2, 1), Cells(3, baseCol + codeCalcNum * stepCol - 1)), PlotBy:=xlRows
    .Range(Cells(3, 1), Cells(3, baseCol + codeCalcNum * stepCol - 1)).Copy
    performChart.chart.SeriesCollection.Paste
    performChart.chart.SeriesCollection(1).XValues = .Range(Cells(2, baseCol), Cells(2, baseCol + codeCalcNum * stepCol - 1))
    
    .Range(Cells(5, 1), Cells(5, baseCol + codeCalcNum * stepCol - 1)).Copy
    performChart.chart.SeriesCollection.Paste
    performChart.chart.SeriesCollection(2).XValues = .Range(Cells(2, baseCol), Cells(2, baseCol + codeCalcNum * stepCol - 1))
    
    ' タイトルを設定
    performChart.chart.HasTitle = True
    performChart.chart.ChartTitle.Text = "ほげほげ：" + CStr(ir * 100) + "％"
    performChart.chart.Axes(xlCategory).HasTitle = True
    performChart.chart.Axes(xlCategory).AxisTitle.Text = .Cells(2, 1).Value
    performChart.chart.Axes(xlValue).HasTitle = True
    performChart.chart.Axes(xlValue).AxisTitle.Text = .Cells(3, 1).Value
End With

こうすると正しく動作することがわかりました．
何なんだよ，いったい．もう疲れたよ．．．
でも，朝までには完成させないとなぁ．．．