Since you already know where you want to break the text, you could consider just adding separate elements.
.append("text")
.attr("dy", "0em")
.text("line 1") // "line 1" or whatever value you want to add here.
And then for the second line, just add an offset
.append("text")
.attr("dy", "1em") // you can vary how far apart it shows up
.text("line 2") // "line 2" or whatever value you want to add here.
