Add gradient or transparency changes for line objects

I think this would be a great addition, as allows further customization in data viewing. For example, in my application I need to display strain and other variables during charging and discharging cycles of a battery, and in my current plot the starting and ending point are not distinguishable as I'm using different colors for different variables, different shades for different cycles, and different dash styles for different channels. I would like the ability to have a gradient or a transparency based on another variable (in my case timestamps) to allow additional data to be shown., also with different RGBA values for starting and ending point.

Issue 中没有确定文件、测试或入口点。首先定位 line-object API 及其现有的颜色处理，然后明确渐变或透明度应如何由另一个变量驱动，以及哪些 RGBA 值定义端点，之后再确定所需的测试。