Problem with graphing an array

I must not have a full understanding of how to work with some of the graphic functions and need your help. I have included my code with 3 pics. Pic 1 is when I plot the portion of the graph I am working with, pic 2 is the successful plotting of the parabola pic 3 shows the problem. When I click on the first button it displays the graph paper which works fine. Then when I click on the graph 1 button it plots out the parabola which also works fine. However if I click the graph 1 button again it adds a line connecting the 2 points of the parabola which I have no idea where this extra line is coming from. If someone can help me I would really appreciate it.

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Uses a form, 3 buttons and a picturebox

Imports System.Drawing.Drawing2D  
  
Public Class Form4  
    Private bmp As Bitmap = Nothing  
    Private gPath As New GraphicsPath()  
    Dim scaleX As Single = 1  
    Dim scaleY As Single = 1  
    Dim magnifiery As Integer  
    Dim g As Graphics  
    Private _d As Double = 0  
    Private Sub Button3_Click(sender As Object, e As EventArgs) Handles Button3.Click  
        PlotGraphPaper(_d)  
    End Sub  
  
    Private Sub PlotGraphPaper(ByVal d As Double)  
        Dim t, dumX, dumY As Single  
        Dim i As Integer = 0  
        dumX = CSng(ClientSize.Width / 2)  
        dumY = CSng(ClientSize.Height / 2)  
        bmp = New Bitmap(Me.ClientSize.Width, Me.ClientSize.Height)  
        PictureBox1.Width = bmp.Width  
        PictureBox1.Height = bmp.Height  
        Using g As Graphics = Graphics.FromImage(bmp)  
            g.Transform = New Matrix(1, 0, 0, 1, CSng(ClientSize.Width / 2), CSng(ClientSize.Height / 2))  
            g.ScaleTransform(scaleX, scaleY)  
            t = CInt((dumX / 40) - 1)  
            While i <= 23 * 40 '< bmp.Width / 2  
                g.DrawLine(Pens.LightGray, New Point(i, CInt(-bmp.Height / 2)), New Point(i, CInt(bmp.Height / 2)))  
                g.DrawLine(Pens.Black, New Point(i, 5), New Point(i, -5))  
                i = i + CInt(t)  
            End While  
            g.SmoothingMode = SmoothingMode.AntiAlias  
            g.DrawPath(Pens.Black, gPath)  
            g.DrawEllipse(Pens.Red, 0 - 5, 0 - 5, 10, 10)  
            PictureBox1.Image = bmp  
        End Using  
    End Sub  
    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click  
        Dim pts As New List(Of PointF)()  
        Dim x As Integer  
        Dim y As Single  
        For x = -175 To 175  
            y = -CSng((x ^ 2) / 100)  
            pts.Add(New PointF(x, y))  
        Next  
        gPath.AddLines(pts.ToArray())  
        Using g As Graphics = Graphics.FromImage(bmp)  
            g.Transform = New Matrix(1, 0, 0, 1, CSng(ClientSize.Width / 2), CSng(ClientSize.Height / 2))  
            g.ScaleTransform(scaleX, scaleY)  
            g.SmoothingMode = SmoothingMode.AntiAlias  
            g.DrawPath(Pens.Black, gPath)  
            PictureBox1.Image = bmp  
        End Using  
    End Sub  
End Class  

Thanks

Les