Questa è la soluzione più semplice ma anche la più "superficiale"

codice:
Imports System
Imports System.Windows.Forms
Imports System.Drawing

Public Class frm_diametri
Private Disegna as Boolean=False

Private Sub frm_diametri_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
        DataGrid1.Columns.Add(1, "X")
        DataGrid1.Columns.Add(2, "Z")
    End Sub

    Private Sub PictureBox1_Paint(ByVal sender As Object, ByVal e As System.Windows.Forms.PaintEventArgs) Handles PictureBox1.Paint
If Disegna Then
        Dim gr As Graphics = e.Graphics
        gr.DrawLine(New Pen(Color.Black), 0, 300, 20, 300)
        gr.DrawLine(New Pen(Color.Black), 20, 300, 20, 200)
Disegna=False
End If
    End Sub

    Private Sub Button3_Click(ByVal sender As System.Object, ByVal d As System.EventArgs) Handles Button3.Click
Disegna=True
        PictureBox1.Refresh()
    End Sub

End Class
Facci sapere...
Ciao