Got the answer myself : inserted 'clear' statement. here's the new code :
Chart31in.Chart1.Series("SP").Points.DataBindXY(Depth1, SP1)
Chart31in.Chart1.Series("SP").Points.Clear()
For i As Long = IndexProcTop To IndexProcBot
Chart31in.Chart1.Series("SP").Points.AddXY(Depth1(i), SP1(i))
Next
Thanks for looking at my question.