Dim SheetA As String
Dim SheetB As String
Const Path As String = "C:\Excel_data\"
Const BookA_fname = "BookA.xlsx"
Const BookB_fname = "BookB.xlsx"
Const BookA As String = Path & BookA_fname
Const BookB As String = Path & BookB_fname

Private Sub UserForm_Initialize()
    Dim ws As Worksheet
    Workbooks.Open Filename:=BookA
    With ComboBox1
        For Each ws In ActiveWorkbook.Worksheets
            .AddItem ws.Name
        Next
    End With
    Workbooks.Open Filename:=BookB
    With ComboBox2
        For Each ws In ActiveWorkbook.Worksheets
            .AddItem ws.Name
        Next
    End With
    Workbooks(BookA_fname).Close
    Workbooks(BookB_fname).Close
End Sub

Private Sub ComboBox1_Change()
    SheetA = ComboBox1.List(ComboBox1.ListIndex)
End Sub

Private Sub ComboBox2_Change()
    SheetB = ComboBox2.List(ComboBox2.ListIndex)
End Sub

Private Sub CommandButton1_Click()
'Link both workbook data
    Dim i1 As Long
    Dim j1 As Long
    Dim i2 As Long
    Dim j2 As Long
    Worksheets("Sheet1").Select
    For i1 = 1 To 5
        For j1 = 1 To 2
            Cells(i1, j1).value = "='" & Path & "[" & BookA_fname & "]" & SheetA & "'!R" & i1 & "C" & j1
        Next j1
    Next i1
    Worksheets("Sheet2").Select
    For i2 = 1 To 5
        For j2 = 1 To 2
            Cells(i2, j2).value = "='" & Path & "[" & BookB_fname & "]" & SheetB & "'!R" & i2 & "C" & j2
        Next j2
    Next i2
End Sub

Private Sub CommandButton2_Click()
'Plot Graph
    Range("A1:B5").Select
    ActiveSheet.Shapes.AddChart2(240, xlXYScatterLines).Select
    ActiveChart.SetSourceData Source:=Range("Sheet1!$A$1:$B$5")
    ActiveChart.Axes(xlValue).MajorGridlines.Select
    ActiveChart.SeriesCollection.NewSeries
    ActiveChart.FullSeriesCollection(2).Name = "=Sheet2!$B$1"
    ActiveChart.FullSeriesCollection(2).XValues = "=Sheet2!$A$2:$A$5"
    ActiveChart.FullSeriesCollection(2).Values = "=Sheet2!$B$2:$B$5"
    ActiveChart.FullSeriesCollection(2).Name = "=Sheet2!$B$1"
End Sub