add a progress gauge for plotting progress

It looks like there is no way to show to the user how many of the total points have been plotted, and plotly completely locks the web page, preventing any update until script finishes!
I found only the event triggered at the end of plot, but it's useless: once the plot is ended... user can see the plot!
We'd need an event fired at each point plotted, or something like that.

未指定任何文件或测试。首先跟踪绘图生命周期以及绘图结束时触发的现有事件；明确如何在绘图仍在运行时公开进度更新，并考虑报告的页面锁定问题。完成的标准是：有一项文档化且可测试的进度信号，在绘图完成之前更新，同时不会让页面无响应。