说明：  一个图中有能画多个曲线，而且线的颜色随变量变化而颜色变化，图例也是随变量不同命名也是不同的！非常经典的图，值得学习。
(There can draw a graph multiple curves, and color of the line changes with variable color change, the legend is different with the variable name is different! Very classic diagram, it is worth learning.)