an example to reproduce the `Cannot read property '0' of null in clipCells()` error message described in [d3-voronoi github issue 16](https://github.com/d3/d3-voronoi/issues/16)

this example uses `d3.verbose.js`, which is a copy of `d3.js` v4.2.3 with some logging added to help trace the error message.

see earlier examples: 

- working, with another column for x [Voronoi clipCells() end[1] null Example](http://bl.ocks.org/micahstubbs/2e7c882e69aa10d9dc317b59baf64692)  
- not working, error reproduced [Voronoi clipCells() end[1] null Example II](http://bl.ocks.org/micahstubbs/ac0faf07e5c83e97003b5a45e437e767)  

the code for the `d3-voronoi-scatterplot` chart plugin is at [https://github.com/micahstubbs/d3-voronoi-scatterplot](https://github.com/micahstubbs/d3-voronoi-scatterplot)