c#-C#畫圖：用C#畫圖的時候遇到了問題

C#畫圖：用C#畫圖的時候遇到了問題

 using System;
using System.Data;
using System.Configuration;
using System.Collections;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using System.Text;
using System.Drawing;
using System.Text.RegularExpressions;
using System.Reflection;
using System.Collections.Generic;
using Tools;
using DBAPI;
using Functions;
using System.Web.Script.Serialization;
using System.IO;//文件存取
using System.Drawing;//畫圖基本功能
using System.Drawing.Drawing2D;//二維畫圖
using System.Drawing.Imaging;//高級功能

public partial class V2_NodeState_AirCleaner2_Chart2 : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        Bitmap img = new Bitmap(400, 200);//創建Bitmap對象
        MemoryStream stream = draw();

        img.Save(stream, ImageFormat.Jpeg);          //保存繪制的圖片
        Response.Clear();
        Response.ContentType = "image/jpeg";
        Response.BinaryWrite(stream.ToArray());
    }
    public MemoryStream draw()
    {
        Bitmap img = new Bitmap(400, 200);//創建Bitmap對象
        Graphics g = Graphics.FromImage(img);//創建Graphics對象

        Pen Bp = new Pen(Color.Black); //定義黑色畫筆
        Pen Rp = new Pen(Color.Red);//紅色畫筆
        Pen Sp = new Pen(Color.Blue);//藍色
        SolidBrush redBrush = new SolidBrush(Color.Red);
        AdjustableArrowCap aac;  //定義箭頭帽

        aac = new System.Drawing.Drawing2D.AdjustableArrowCap(4, 4);
        Sp.CustomStartCap = aac;  //開始端箭頭帽
        //Sp.CustomEndCap = aac;

        Font Bfont = new Font("Arial", 12, FontStyle.Bold);//大標題字體
        Font font = new Font("Arial", 6);//一般字
        Font Tfont = new Font("Arial", 9);//較大字體

        float w = 1.5F;
        float h = 1.5F;
        g.DrawRectangle(new Pen(Color.White, 400), 0, 0, img.Width, img.Height); //矩形 底色


        //黑色過度型筆刷
        LinearGradientBrush brush = new LinearGradientBrush(new Rectangle(0, 0, img.Width, img.Height), Color.Black, Color.Black, 1.2F, true);


        //藍色過度型筆刷
        LinearGradientBrush Bluebrush = new LinearGradientBrush(new Rectangle(0, 0, img.Width, img.Height), Color.Blue, Color.Blue, 1.2F, true);

        //g.DrawString("我的實驗",Bfont,brush,40,5); //繪制大標題

        g.DrawRectangle(Bp, 0, 0, img.Width - 100, img.Height - 40); //繪制矩形與上面的矩形即是邊框

        //原點坐標（150,90），一個單位坐標=8px
        g.DrawLine(Sp, 150, 10, 150, 150);//縱坐標
        g.DrawLine(Sp, 300, 90, 20, 90);//橫坐標


        g.DrawString("5", font, brush, 188, 91); //橫坐標數字
        g.DrawString("10", font, brush, 226, 91);
        g.DrawString("15", font, brush, 266, 91);

        //g.DrawString("0.5", font, brush, 135, 86); //縱坐標數字
        g.DrawString("1", font, brush, 135, 78);
        //g.DrawString("1.5", font, brush, 135, 78);
        g.DrawString("2", font, brush, 135, 70);
        g.DrawString("3", font, brush, 135, 62);

        g.DrawLine(Bp, 190, 85, 190, 90); //橫坐標刻度線
        g.DrawLine(Bp, 230, 85, 230, 90);
        g.DrawLine(Bp, 270, 85, 270, 90);

        g.DrawLine(Bp, 155, 82, 150, 82);//縱坐標刻度線
        g.DrawLine(Bp, 155, 74, 150, 74);
        g.DrawLine(Bp, 155, 66, 150, 66);

        //畫函數曲線   這樣以   y=x^2  為例   用窮舉函數曲線中的兩點畫曲線
        //畫0-2中的點，步長為0.1
        for (double i = 0; i <= 2; )
        {
            double Write11 = (150 + 8 * i);
            double Write12 = 90 - 8 * (Math.Pow(i, 2));

            double Write21 = (150 + 8 * (i + 0.1));
            double Write22 = 90 - 8 * (Math.Pow(i + 0.1, 2));

            g.DrawLine(Bp, (float)Write11, (float)Write12, (float)Write21, (float)Write22);
            //img.SetPixel((int)Write11, (int)Write12, Color.Red);
            g.FillEllipse(redBrush,(float)Write11,(float)Write12, w, h);


            i = i + 0.1;
        }

        for (double i = -2; i <= 0; )
        {
            double Write11 = (150 + 8 * i);
            double Write12 = 90 - 8 * (Math.Pow(i, 2));

            double Write21 = (150 + 8 * (i + 0.1));
            double Write22 = 90 - 8 * (Math.Pow(i + 0.1, 2));

            g.DrawLine(Bp, (float)Write11, (float)Write12, (float)Write21, (float)Write22);
            i = i + 0.1;
        }

        MemoryStream stream = new MemoryStream();   //保存繪制的圖片
        img.Save(stream, ImageFormat.Jpeg);          //保存繪制的圖片
        return stream;
    }

}

 Bitmap img = new Bitmap(1000, 500);//創建Bitmap對象
將new Bitmap(400, 200)中的值修改為（1000，500）就會在圖片中出現一個黑色的矩形框
而且經過嘗試，發現只要改大，就會出現黑色的框，同時這個框也會隨著修改的值而變。

如下圖所示：

最佳回答：

下面這句話引起的，你以400的線粗畫了個矩形，但是沒有fill. 中間就是黑的類，bitmap尺寸小的時候，線把所有bitmap的地方都占據了，所以沒有問題
g.DrawRectangle(new Pen(Color.White, 400), 0, 0, img.Width, img.Height); //矩形 底色

要填充底色用下面的語句
g.FillRectangle(new SolidBrush(Color.White), 0, 0, img.Width, img.Height);