Thanks Max, it worked !

here is the vb.net code I used (I added statement for 2 times wraping also):

  Dim labels As New Hashtable
  labels.Add("LABELS", New Labels)
  Me.UltraChart1.LabelHash = labels
  Me.UltraChart1.Legend.FormatString = "<LABELS>"
 

Public Class Labels
    Implements IRenderLabel

    Public Overloads Function ToString(ByVal context As System.Collections.Hashtable) As String Implements Infragistics.UltraChart.Resources.IRenderLabel.ToString
        Dim text As String = CType(context("ITEM_LABEL"), String)
        If text.Length > 20 Then
            text = text.Insert(20, System.Environment.NewLine)
            text = text.Insert(10, System.Environment.NewLine)
        ElseIf text.Length > 10 Then
            text = text.Insert(10, System.Environment.NewLine)
        End If
        Return text
    End Function
End Class