Customizing the legend manually.

[AndreiKingsley]

At the moment, the legends are based on mappngs/scales. Beyond that, however, it would be nice to be able to create a legend completely manually, without being tied to other elements of the plot.

For example, I can build the next plot:

And I would like to add to it a legend like

<blue line> "A"
<red point> "B"

I can trick lets-plot the following way:

However it doesn't work well for other aesthetic attributes, so I would like to have a complete customization of the legend, regardless of data/layers/mapping/scales, etc.