composite leaflet chart

[gordonwoodhull]

Someone could fairly easily implement a composite leaflet chart - it wouldn't share any code with (dc.compositeChart)[https://github.com/dc-js/dc.js/blob/develop/web/docs/api-latest.md#dc.compositeChart] and very little interface, but the same concept of the composite chart owning the leaflet instance and the children drawing layers within it.

[gordonwoodhull]

Note that it requires changing the individual charts just a little bit - they have to notice that they have a parent, and rely on the parent to do the top-level initialization.

It's not a perfect design but it would be a step forward.