Hi,

I added a line of code to activate sheet3.

Private Sub OKButton_Click()

    Dim intLbxIndex As Integer    'ListBox (Lbx) index number
    Dim rngCell As Range, rngMyRange As Range
    Dim rngChtData As Range
    Dim rngChtXVal As Range
    Dim iColumn As Long
    Dim theChart As Chart
    Dim FirstSeries As Boolean
    Dim max_cyc As Integer

    Dim activsht As Variant
    ' check if worksheet or chart sheet and delete if they do
    Application.DisplayAlerts = False
    For Each activsht In ActiveWorkbook.Sheets
        If activsht.Name = "Current Waveforms" Then
            activsht.Delete
        ElseIf activsht.Name = "Voltage Waveforms" Then
            activsht.Delete
        End If
    Next activsht
    Sheets("Analog_Data").Activate''''Added this line
    Set rngChtXVal = Range(Sheet3.Cells(2, 1), Sheet3.Cells(Sheet3.Cells(Rows.Count, 1).End(xlUp).Row, 1))
    max_cyc = Sheet1.Range("D6").Value
    ' Make the line graph for current
    FirstSeries = True
    For intLbxIndex = 0 To ListBox2.ListCount - 1
        For Each rngCell In Sheet3.Range("B1:J1")
            If (CStr(rngCell.Value) = CStr(ListBox2.List(intLbxIndex)) _
                And Mid(CStr(rngCell.Value), 1, 1) = "I") Then

                ' add the chart when the first series is encountered
                If (FirstSeries) Then
                    FirstSeries = False
                    Set theChart = Charts.Add
                    With theChart
                        .ChartType = xlXYScatterSmoothNoMarkers
                        .Name = "Current Waveforms"
                        .Axes(xlCategory).MaximumScale = max_cyc
                        .Move After:=Sheets(Sheets.Count)
                    End With
                    With theChart.Axes(xlCategory, xlPrimary)
                        .HasTitle = True
                        With .AxisTitle
                            .Characters.Text = "Time (Cycles)"
                        End With
                    End With
                    With theChart.Axes(xlValue, xlPrimary)
                        .HasTitle = True
                        With .AxisTitle
                            .Characters.Text = "Current (A)"
                        End With
                    End With
                    ' remove any series already place on the chart
                    Do Until theChart.SeriesCollection.Count = 0
                        theChart.SeriesCollection(1).Delete
                    Loop
                End If

                ' add this series to the chart
                iColumn = rngCell.Column
                Set rngChtData = Range(Sheet3.Cells(2, iColumn), Sheet3.Cells(Sheet3.Cells(Sheet3.Rows.Count, iColumn).End(xlUp).Row, iColumn))
                With theChart.SeriesCollection.NewSeries
                    .Values = rngChtData
                    .XValues = rngChtXVal
                    .Name = Sheet3.Cells(rngChtData.Row - 1, rngChtData.Column)
                End With
            End If
        Next rngCell
    Next intLbxIndex

    ' Make the line graph for voltage
    FirstSeries = True
    For intLbxIndex = 0 To ListBox2.ListCount - 1
        For Each rngCell In Sheet3.Range("B1:J1")
            If (CStr(rngCell.Value) = CStr(ListBox2.List(intLbxIndex)) _
                And Mid(CStr(rngCell.Value), 1, 1) = "V") Then

                ' add the chart when the first series is encountered
                If (FirstSeries) Then
                    FirstSeries = False
                    Set theChart = Charts.Add
                    With theChart
                        .ChartType = xlXYScatterSmoothNoMarkers
                        .Name = "Voltage Waveforms"
                        .Axes(xlCategory).MaximumScale = max_cyc
                        .Move After:=Sheets(Sheets.Count)
                    End With
                    With theChart.Axes(xlCategory, xlPrimary)
                        .HasTitle = True
                        With .AxisTitle
                            .Characters.Text = "Time (Cycles)"
                        End With
                    End With
                    With theChart.Axes(xlValue, xlPrimary)
                        .HasTitle = True
                        With .AxisTitle
                            .Characters.Text = "Voltage (kV)"
                        End With
                    End With
                    ' remove any series already place on the chart
                    Do Until theChart.SeriesCollection.Count = 0
                        theChart.SeriesCollection(1).Delete
                    Loop
                End If
                ' add this series to the chart
                iColumn = rngCell.Column
                Set rngChtData = Range(Sheet3.Cells(2, iColumn), Sheet3.Cells(Sheet3.Cells(Sheet3.Rows.Count, 1).End(xlUp).Row, iColumn))
                With theChart.SeriesCollection.NewSeries
                    .Values = rngChtData
                    .XValues = rngChtXVal
                    .Name = Sheet3.Cells(rngChtData.Row - 1, rngChtData.Column)
                End With
            End If
        Next rngCell
    Next intLbxIndex

    Unload Me
End Sub