matlab帮助文档原文如下：
hold ON holds the current plot and all axis properties, including the current color and linestyle, so that subsequent graphing commands add to the existing graph without resetting the color and linestyle. hold OFF returns to the default mode whereby PLOT commands erase the previous plots and reset all axis properties before drawing new plots.
对应翻译
hold on：保持当前绘图和所有轴属性（包括当前颜色和线型），以便随后的绘图命令添加到现有图形中，而不重置颜色和线型。hold off：返回默认模式，借此PLOT命令擦除前面的绘图和在绘制新图之前重置所有座标轴属性