Compatibility with existing add-on libraries

[gordonwoodhull]

It is possible that #1729 Concept of Chart Group could break compatibility with external libraries that add new charts to the ecosystem, like dc-addons or our own dc.leaflet.js.

Many of these were never updated to D3v4, so it’s not a big deal. But if they were, we shouldn’t break them now.

It looks like the change is probably backward compatible; this issue is a reminder to self to test and be sure.

There are two cases to consider:

• charts that use BaseMixin
• charts that start from scratch

[gordonwoodhull]

I don't think dc-addons has been updated in a while.

I maintain dc.leaflet.js, however. For the dc@4 update I adapted the code without moving to ES6, partly as a POC. Would be glad to move to ES6 & Rollup for the next update. The library doesn't have tests, just the demo page, but otherwise it should be straightforward.

[kum-deepak]

That makes sense, we will start with moving dc.leaflet.js to ES6/typescript and provide assistance for the others as per demand.