zed graph + affichage de données

salut,
j'ai une zedgraphcontrol dans laquel je desire affiché le nom des PC par date ( cad en Xaxis des dates et en Yaxis les nom des pc)
Mon probléme est le suivant je désire affiché sous forme de barcharts sauf que les paramétres ont pour type des string (le nom du pc) et non pas des doubles , je fais quoi?
Code:
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 Try
            Dim a As New Métier.alerte
 
            Dim requete As String = "SELECT  COUNT(*) AS  Nombre  FROM alerte where date_alerte >  '" & DateTimePicker1.Value.ToString("MM/dd/yyyy ") & "' and date_alerte <'" & DateTimePicker2.Value.ToString("MM/dd/yyyy ") & "' "
            Dim reader1 As SqlClient.SqlDataReader = a.consulter_alerte(requete)
            Dim nombre As Int32
 
            While reader1.Read
 
                nombre = reader1.GetInt32(0)
            End While
 
            Dim tab(nombre) As String
            Dim t(nombre) As String
 
            Dim im As New images
 
            Dim myPane As ZedGraph.GraphPane = im.ZedGraphControl1.GraphPane
            ' Set the titles and axis labels
            myPane.Title.Text = "Statistiques sur les toutes les pannes"
            myPane.XAxis.Title.Text = " période"
            myPane.YAxis.Title.Text = "Pannes(PC)"
            Dim connectString As String = "Data Source=SARA\GMAO;Initial Catalog=GMAO;User ID=sa; password = 21009324"
            'Objet connection
            Dim connection As SqlClient.SqlConnection = New SqlClient.SqlConnection(connectString)
 
            '--------------------------------------------------------------------------------------
            'Ouverture
            connection.Open()
            Dim i As Integer = 0
            Dim command1 As SqlClient.SqlCommand = New SqlClient.SqlCommand("SELECT id_pc_testeur , date_alerte FROM alerte where date_alerte >  '" & DateTimePicker1.Value.ToString("MM/dd/yyyy ") & "' and date_alerte <'" & DateTimePicker2.Value.ToString("MM/dd/yyyy ") & "' ", Connection)
            Dim reader As SqlClient.SqlDataReader = command1.ExecuteReader
            While reader.Read
                t(i) = reader.GetString(0)
                tab(i) = reader.GetString(1)
 
 
 
                ' Set the XAxis labels
                myPane.XAxis.Scale.TextLabels = tab
                ' Set the XAxis to Text typeab
                myPane.XAxis.Type = AxisType.Text
 
                myPane.YAxis.Scale.TextLabels = t
                myPane.YAxis.Type = AxisType.Text
 
                ' Generate a red bar with "Curve 1" in the legend
                Dim myBar As BarItem = myPane.AddBar("Pc testeur", Nothing, t(i), Color.DarkSlateBlue)
 
                myBar.Bar.Fill = New Fill(Color.SkyBlue, Color.White, Color.Purple)
 
                ' Draw the X tics between the labels instead of at the labels
                myPane.XAxis.MajorTic.IsBetweenLabels = True
 
                ' Fill the axis background with a color gradient
                myPane.Chart.Fill = New Fill(Color.White, Color.LightGoldenrodYellow, 45.0F)
 
                ' Fill the pane background with a color gradient
                myPane.Fill = New Fill(Color.White, Color.FromArgb(220, 220, 255), 45.0F)
                myBar.Bar.Fill.RangeMin = 0
                myBar.Bar.Fill.RangeMax = 4
                i = i + 1
            End While
  reader.Close()
 
 
            ' Calculate the Axis Scale Ranges
            im.ZedGraphControl1.AxisChange()
            im.ShowDialog()
 
 
            ' connection.Close()
        Catch
            MsgBox("ooooooops", MsgBoxStyle.OkOnly)
        End Try
Merci