Chart legends are not focusable

NVD3 version used: 1.8.6

Browser and OS used: Chrome Mac OS

Live Example: https://jsfiddle.net/zac1st1k/t8d3n57m/68/

Expected Behaviour: Line 1 and Line 2 legends can gain focus by pressing TAB key.

Present Behaviour: Line 1 and Line 2 legends are not keyboard interactive.

Screenshot: https://bugzilla.mozilla.org/show_bug.cgi?id=1302341

Related doc: https://allyjs.io/tutorials/focusing-in-svg.html