Option Infer On
Public Class frmLinqGroup
    Dim dt As New DataTable
 
    Dim cht As New System.Windows.Forms.DataVisualization.Charting.Chart With
    {.Parent = Me, .Size = New Size(400, 400)}
 
 
    Private Sub frmLinqGroup_Load(sender As Object,
                                  e As EventArgs) Handles MyBase.Load
 
        'Testdaten
        dt.Columns.Add("Datum", GetType(Date))
        dt.Columns.Add("Anwesend", GetType(Integer))
 
        Dim basedate As Date = CDate("2.5.2016")
        Dim minutes() As Integer = {320, 410, 490}
        Dim rndm As New Random(12345)
 
        For i As Integer = 0 To 1000
            Dim dtx As Date = basedate.AddDays(i)
            Dim min As Integer = minutes(rndm.Next(0, 3))
            If dtx.DayOfWeek.Equals(DayOfWeek.Saturday) OrElse
               dtx.DayOfWeek.Equals(DayOfWeek.Sunday) Then min = 0
            dt.Rows.Add(dtx, min)
        Next i
 
        'Gruppieren der Anwesenheitstage pro Monat        
        Dim q = From row As DataRow In dt.AsEnumerable
                Let mnt As String = DateAndTime.MonthName(DirectCast(row( _
                  "Datum"), Date).Month)
                Let yr As Integer = DirectCast(row("Datum"), Date).Year
                Let anw As Boolean = DirectCast(row("Anwesend"), Integer) > 0
                Where anw
                Group By yr, mnt Into Group, Count()
 
 
        'optional: Aufbereitung der Abfrage als Tabelle
        Dim dtb As New DataTable
        dtb.Columns.Add("year-mnt", GetType(String))
        dtb.Columns.Add("count", GetType(Integer))
 
        For Each at In q
            With at
                'anonymer Typ in Datatable-Zeile, Jahr-Monat zusammenbauen
                dtb.Rows.Add(.yr.ToString & "-" & .mnt, .Count)
            End With
        Next at
 
        'Einrichtung des Chart
        With cht
            .DataSource = dtb
            .ChartAreas.Add("Base")
            .ChartAreas("Base").AxisX.Interval = 1
 
            .DataSource = dtb
 
            .Series.Add("Anw")
            With .Series("Anw")
                .XValueMember = "year-mnt"
                .YValueMembers = "Count"
                .ChartArea = "Base"
                .ChartType = DataVisualization.Charting.SeriesChartType.Bar
            End With
 
            .Titles.Add("Anwesenheitstage pro Monat")
 
        End With
    End Sub
End Class

     Dim dt As DataTable = AblageDB.Arbeitszeiten
        Dim year_selected As Integer = CInt(cboYear.SelectedItem)
        'Häufigkeitstabelle aus Datensätzen erstellen
        Dim dc As New DataTable
        With dc.Columns
            .Add("Month", GetType(String))
            .Add("Freq. done", GetType(Integer))
            .Add("Freq. todo", GetType(Integer))
        End With
 
        dc.PrimaryKey = {dc.Columns("Month")}
 
        For i As Integer = 0 To dt.Rows.Count - 1
            Dim dr As DataRow = dt.Rows(i)
            If DirectCast(dr("Tag"), Date).Year = year_selected Then
                'sortierbarer Jahr-Monat-String
                Dim ym As String = _
                 dr.Field(Of Date)("Tag").Year.ToString & "-" & _
                 dr.Field(Of Date)("Tag").Month.ToString("00")
 
                Dim el As String = dr.Field(Of String)("Bemerkung")
 
                'Datensatzsuche nach Monat in Häufigkeitstabelle
                'gemäß Primärschlüssel     
                Dim dcr As DataRow = dc.Rows.Find({ym})
                If dcr Is Nothing Then
                    dcr = dc.Rows.Add({ym, 0, 0})
                End If
 
                Dim fcol As String
                If el = "Anwesend" Then
                    fcol = dc.Columns(1).ColumnName
                Else
                    fcol = dc.Columns(2).ColumnName
                End If
 
                'Häufigkeit bestimmen
                dcr(fcol) = CInt(dcr(fcol)) + 1
            End If
        Next i
 
        Dim bs As New BindingSource With {.DataSource = dc}
        bs.Sort = "Month ASC"
 
        'Chart für Häufigkeiten erstellen
 
        End With

  Private Sub An_Abwesenheit_User_fill()
        dgv_AZ_GLZ_Auswertung.Rows.Clear()
        'col_SB_Stoerdauer_Maschine
        Dim Abfrage = From Eintrag In AblageDB.Arbeitszeiten
                      Where Eintrag.RowState <> DataRowState.Deleted
                      Order By Eintrag.User
                      Group By Art = Eintrag.UserID Into UGruppe = Group
 
        Dim User As String
        Dim gleitzeit As Double
        Dim D_von, D_bis As Date
 
        Dim ue As ArbeitszeitenRow
        For Each Reihe In Abfrage
            ue = Reihe.UGruppe.First
 
            D_von = (From drv In Reihe.UGruppe Select drv.Tag).Min
            D_bis = (From drv In Reihe.UGruppe Select drv.Tag).Max
 
            Dim rowUser = AblageDB.Arbeitszeiten.First(Function(f) f.ID = _
              ue.UserID)
            If Not rowUser Is Nothing Then
                User = rowUser.User
                gleitzeit = Reihe.UGruppe.Sum(Function(f) f.Gleitzeit)
 
                dgv_AZ_GLZ_Auswertung.Rows.Add({ue.UserID, ue.User, _
                  Reihe.UGruppe.Count, gleitzeit, D_von, D_bis})
            End If
        Next
    End Sub

  Dim Abfrage = _
           From row In Me.AblageDB.Arbeitszeiten.AsEnumerable _
           Let Jahr = CInt(row.Field(Of Date)("Tag").ToString("yyyy")) _
           Order By row.Tag _
           Group row By Jahr Into Group _
           Select Jahr, Group, _
           Gleitzeitminuten = Group.Sum(Function(row) row.Gleitzeit), _
           Arbeitszeitminuten = Group.Sum(Function(row) row.IstAZ)