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 current axes to add graphics, or set the current axes properties NexPlot for the add,