Add support for clickable hover text or a per pt onclick attribute

Users can put links inside hover labels, but when user try to click the link, the hover label obviously disappears. So perhaps we could add optional “sticky” hover labels, where you click and the hover label is persistent until you click again?

Defining what sticky means will be tricky. It probably doesn't apply to every situation. Maybe we could add it as a new layout.hovermode e.g. 'hovermode: 'sticky'.

So, maybe instead we could define a linkonclick attribute e.g. linkonclick: ['https://plot.ly/~empet/101', 'https://plot.ly', /* ... one link per data pt */] that way clicking on a data pt would open the link instead of hovering on the data pt -> move mouse over link in hover label -> click on it.

cc @jackparmer

未指定檔案、測試或進入點。首先釐清預期行為是固定顯示的 hover 標籤，還是每個點在點擊時對應的連結；只有在互動定義確定且實作與其一致後，才算完成。