D3 5.16.0: data.labels.format regression

Found a regression in [data.labels.format](https://c3js.org/reference.html#data-labels-format):
The j parameter used to be the sub index of the data point but is now an array of SVGTextElement. Might be a regression due to the new d3.js version.
Below the CodePen showing the issue.

Before (D3 3.5.16): https://codepen.io/florentmontesano/pen/PoxoBYG
After (D3 5.16.0): https://codepen.io/florentmontesano/pen/jOQOKjG

* **C3 version**: 0.7.20
* **D3 version**: 5.16.0
* **Browser**: Chrome 114.0.5735.110
* **OS**: Windows 10