Chart reset problem

Hi.

I'm a beginner of VB.net and tried to build a line chart. Every time I run the code,it seems that chartcontrol doesn't refresh itself,just keep print the title.Here's my code:

Public Sub ChartControl1_Initialize()

    Dim Series As ChartSeries
    Set Series = ChartControl1.Content.Series.Add("New York")
    ...
    ChartControl1.Content.Titles.Add "Population by U.S. State"
    ChartControl1.Content.Legend.Visible = True
    ...

End Sub

And if I run the program 5 times,it looks like:

Population by U.S. State
Population by U.S. State
Population by U.S. State
Population by U.S. State
Population by U.S. State
[chart]

Is there any code to reset the chartcontrol?Or just because I wrote the code in a wrong way?