Make root and svg read only

[kum-deepak]

The current API allows these to be updated. The documentation warns against that. Charts are lilely to be messed up if someone does set these. We should make these read-only with backward compatibility.

@gordonwoodhull, waiting for your views.

[gordonwoodhull]

I disagree. We don't protect users from themselves in this library. It's a leaky abstraction by design.

[kum-deepak]

Since we moved to ES6, accessing any property is possible, which the current version will continue to allow. Through documented API, I think that we should allow read-only access to these properties.