I agree with you that is his personal opinion about "preferable" way to use D3.
> "Unfortunately, I always find D3 code surprisingly difficult to understand and extend"
I would say something different - it is very easy to extend and create various visualization. It is easy to scaffold new chart (line, pie, box-plot, heatmap etc.) and customize it to specified needs. I'd rather say that it is difficult to create a brand new type of visualization which wasn't been already developed [1] and it is easy to understand by random folk.
> "Unfortunately, I always find D3 code surprisingly difficult to understand and extend"
I would say something different - it is very easy to extend and create various visualization. It is easy to scaffold new chart (line, pie, box-plot, heatmap etc.) and customize it to specified needs. I'd rather say that it is difficult to create a brand new type of visualization which wasn't been already developed [1] and it is easy to understand by random folk.
[1] https://github.com/d3/d3/wiki/Gallery