matlab 常用命令，如：hold on 用于保留当前图形和当前坐标轴属性，后面的图形命令只能在当前存在的坐标轴中增加图形，即设置当前坐标轴属性NexPlot为add，-matlab commonly used commands, such as: hold on to retain the current graphics and the current axes properties, behind the graphics commands only exist in the curre