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 では、ファイル、テスト、エントリーポイントが特定されていません。まず line-object API と既存の色処理を見つけ、次にグラデーションや透明度を別の変数でどのように制御するべきか、またどの RGBA 値が端点を定義するのかを明確にしてから、必要なテストを決定してください。