Fix legend width bug introduced by 6752bf5c

A commit https://github.com/novus/nvd3/commit/6752bf5cc831e69e17572e91c3a8958ede8996aa removes required `.node()`
```
-                nodeTextLength = legendText.node().getComputedTextLength();
+                nodeTextLength = legendText.getComputedTextLength();
```

Due to this, error always occurs and `nv.utils.calcApproxTextWidth(legendText);` is used.

This fix may resolve recent comment on #107
Please take a look.