io farei così:
salverei il grafico come immagine, includerei l'immagine come attach nella mail per poi visualizzarla nel codice HTML.

Ecco il codice di esempio:
codice:
        /////////////////////////////////////////////////////////////////////
        // Build a mail object.
        // 

        MailMessage mail = new MailMessage();
        mail.To.Add("anyreceiver@anydomain.com");
        mail.From = new MailAddress("anyaddress@anydomain.com");
        mail.Subject = "Test email - CSSMTPSendEmail";
        mail.Body = "Welcome";
        mail.IsBodyHtml = true;
        
        // Attachments
        string attachedFile = "<attached file path>";
        mail.Attachments.Add(new Attachment(attachedFile));

        // Embedded image in the message body
        mail.Body += "
<img alt=\"\" src=\"cid:image1\">";

        string imgFile = "<image file path>";
        AlternateView htmlView = AlternateView.CreateAlternateViewFromString(
            mail.Body, null, "text/html");
        LinkedResource imgLink = new LinkedResource(imgFile, "image/jpg");
        imgLink.ContentId = "image1";
        imgLink.TransferEncoding = System.Net.Mime.TransferEncoding.Base64;
        htmlView.LinkedResources.Add(imgLink);
        mail.AlternateViews.Add(htmlView);


        /////////////////////////////////////////////////////////////////////
        // Configure the SMTP client and send the email.
        // 

        // Configure the SMTP client
        SmtpClient smtp = new SmtpClient();
        smtp.Host = "smtp.live.com";
        smtp.Credentials = new NetworkCredential(
            "myaccount@live.com", "mypassword");
        smtp.EnableSsl = true;

        // Send the email
        smtp.Send(mail);