:master: lo avevo provato con IE7 e sembrava andare. Adesso ho provato con Opera e va come dici tu. Prova questa modifica.
codice:
<%@ Page Language="VB" Strict="false" %>

<%@ Import Namespace="System.Drawing" %>
<%@ Import Namespace="System.Drawing.Imaging" %>


<script runat="server">
    Sub Page_Load(ByVal sender As Object, ByVal e As EventArgs)
        lbll.Text = "Calcolo della retta dei minimi quadrati "
        Dim grafico As String = Me.Request.QueryString("grafico")
        If grafico IsNot Nothing AndAlso grafico = "1" Then
            creaGrafico()
        End If
    End Sub

    Private valori(5, 5) As Double
    
    Sub creaGrafico()
        
        Dim I, Sx, Sy, Sxy, Sx2, m, q, n As Double
        Dim y01, y02 As Double
        Dim objBitmap As New Bitmap(200, 200)
        Dim objGraphic As Graphics = Graphics.FromImage(objBitmap)
        Dim whiteBrush As New SolidBrush(Color.White)
        Dim redpen As New Pen(Color.Red, 2)
        Dim blackPen As New Pen(Color.Black, 2)
        If Me.Session("valori") Is Nothing Then
            valori(1, 1) = Me.Request.Form("x1")
            valori(1, 2) = Me.Request.Form("y1")
            valori(2, 1) = Me.Request.Form("x2")
            valori(2, 2) = Me.Request.Form("y2")
            valori(3, 1) = Me.Request.Form("x3")
            valori(3, 2) = Me.Request.Form("y3")
            valori(4, 1) = Me.Request.Form("x4")
            valori(4, 2) = Me.Request.Form("y4")
            valori(5, 1) = Me.Request.Form("x5")
            valori(5, 2) = Me.Request.Form("y5")
        
        Else
            valori = DirectCast(Me.Session("valori"), Double(,))
        End If
        n = 5
        Sx = 0
        Sy = 0
        Sx2 = 0
        Sxy = 0
        For I = 1 To 5
            valori(I, 3) = valori(I, 1) * valori(I, 1)
            valori(I, 4) = valori(I, 1) * valori(I, 2)
            Sx = Sx + valori(I, 1)
            Sy = Sy + valori(I, 2)
            Sx2 = Sx2 + valori(I, 3)
            Sxy = Sxy + valori(I, 4)
        Next I
        m = (n * Sxy - Sx * Sy) / (n * Sx2 - (Sx) * (Sx))
        q = (Sx2 * Sy - Sxy * Sx) / (n * Sx ^ 2 - (Sx) * (Sx))
        If q > 0 Then
            lbl2.Text = "Risultato : Y=" & m & "X+" & q
        Else
            lbl2.Text = "Risultato : Y=" & m & "X" & q
        End If
        
        
        y01 = 195 - q
        y02 = 195 - ((195 * m) + q)
        objGraphic.FillRectangle(whiteBrush, 0, 0, 200, 200)
        objGraphic.DrawLine(blackPen, New Point(0, 195), New Point(195, 195))
        objGraphic.DrawLine(blackPen, New Point(5, 5), New Point(5, 200))
        objGraphic.DrawLine(redpen, New Point(5, y01), New Point(195, y02))
        
        If Me.Session("valori") IsNot Nothing Then
            Me.Session("valori") = Nothing
            
            Dim grafico As String = Me.Request.QueryString("grafico")
            If grafico IsNot Nothing AndAlso grafico = "1" Then
                Response.ContentType = "image/gif"
                objBitmap.Save(Response.OutputStream, ImageFormat.Gif)
                Me.Response.End()
            End If
        Else
            Me.Session("valori") = valori
            Me.Literal1.Text = String.Format("[img]a1.aspx?grafico=1&{0}[/img]", DateTime.Now.Ticks)
        End If
    End Sub

    Protected Sub Button1_Click(ByVal sender As Object, ByVal e As System.EventArgs)
        creaGrafico()
    End Sub
    
</script>

<html>
<body>
    <form id="Form1" runat="server">
    


        <asp:Label ID="lbll" runat="server" ForeColor="#0000FF" Font-Bold="true" /></p>
    x1:
    <asp:TextBox ID="x1" Columns="10" runat="server" Text="1" />
    y1:
    <asp:TextBox ID="y1" Columns="10" runat="server"  Text="1" />

    x2:
    <asp:TextBox ID="x2" Columns="10" runat="server" Text="2" />
    y2:
    <asp:TextBox ID="y2" Columns="10" runat="server" Text="4" />

    x3:
    <asp:TextBox ID="x3" Columns="10" runat="server" Text="3" />
    y3:
    <asp:TextBox ID="y3" Columns="10" runat="server" Text="9" />

    x4:
    <asp:TextBox ID="x4" Columns="10" runat="server" Text="4" />
    y4:
    <asp:TextBox ID="y4" Columns="10" runat="server" Text="16" />

    x5:
    <asp:TextBox ID="x5" Columns="10" runat="server" Text="5" />
    y5:
    <asp:TextBox ID="y5" Columns="10" runat="server" Text="25" />

    <asp:Button ID="Button1" runat="server" Text="Button" OnClick="Button1_Click" />
    


        <asp:Label ID="lbl2" runat="server" /></p>
    <asp:Literal ID="Literal1" runat="server"></asp:Literal>
    </form>
</body>
</html>
Se funziona lo spiego, altrimenti... mi nascondo