Искажение результатов построения графика - Visual Basic .NET

  Dim BM As New Bitmap(PictureBox1.Width, PictureBox1.Height)
        Using G As Graphics = Graphics.FromImage(BM)
 
            G.SmoothingMode = Drawing2D.SmoothingMode.AntiAlias
 
            'Рисуем шкалу координат
            'По оcи X
            G.DrawLine(Pens.Black, 0, 130, 550, 130)
            'По оcи Y
            G.DrawLine(Pens.Black, 180, 0, 180, 500)
            'Делаем засечки по осям координат
            'По оси X
            For X = 0 To 550 Step 10
                G.DrawLine(Pens.Black, X, 129, X, 131)
            Next X
 
            'По оси Y
            For Y = 0 To 500 Step 10
                G.DrawLine(Pens.Black, 179, Y, 181, Y)
            Next Y
            Dim brush1 As New SolidBrush(Color.Black)
            Dim font1 As New Font("Arial", 6)
            'Рисуем шкалу координат
            'По оcи X
            For X = -180 To 380 Step 90
                G.DrawString(Math.Round(X * 1.39), font1, brush1, X + 180, 130)
            Next X
            'По оcи Y
            For Y = -150 To 150 Step 50
                G.DrawString(Y / 2, font1, brush1, 180, 130 - Y)
            Next Y
              G.DrawLine(Pens.Red, 180, 130, 180, 131) '0 точка
            'Рисуем график
           For j = 2 To 8
                G.DrawLine(Pens.Red, mat(j) + 180, 70 - mat4(j), mat(j + 1) + 180, 70 - mat4(j + 1)) 'кривая 
            Next j
          PictureBox1.Image = BM
            BM.Save("C:\IS.bmp")
            oSheet = Nothing
            oBook = Nothing
            G.Dispose()
 
        End Using