macro loop through worksheets

After applying macro on Sheet1,go to Sheet2 and to apply the same macro, plse add code

Dim lastrow As Long
Dim selectnum As Long
lastrow = Cells(Rows.Count, "a").End(xlUp).Row
Select Case True
Case lastrow > 30:  selectnum = 30
Case lastrow > 20: selectnum = 20
Case lastrow > 10:   selectnum = lastrow
End Select
Cells(lastrow - selectnum + 1, "a").Resize(selectnum, 3).Select

Set cht1 = ActiveSheet.Shapes.AddChart
    cht1.Chart.ChartType = xlLineMarkers
    cht1.Chart.Location xlLocationAsNewSheet
ActiveChart.ChartArea.Select
    ActiveChart.PlotArea.Select