canvas绘画出现问题，求解答

Question

protected void onDraw(Canvas canvas) {
// TODO Auto-generated method stub
super.onDraw(canvas);

// //画直线
for (int i = 0; i <= 400; i=i+40) {
canvas.drawLine(10, 20+i, 440, 20+i, mPaint);
}

    //绘制点
    mPoint.set(300, 260);
    mPaint.setTextSize(20);
    mPaint.setColor(Color.RED);
    canvas.drawCircle(400, 400,10, mPaint);

    //绘制曲线
    mPath.moveTo(200, 200);
    mPath.lineTo(220, 250);
    canvas.drawPath(mPath, mPaint);

}

XML布局中
android:layout_width="match_parent"
android:layout_height="0dip"
android:layout_weight="2" >

        <com.example.view.MyTemperatureView
            android:id="@+id/m_view"
            android:layout_width="match_parent"
            android:layout_height="match_parent"/>

    </LinearLayout>

为什么只绘画出直线，其他的都画不出来？