I think you're right that there's a quantity of data where using D3.js will start to be more productive than doing it manually in something like Illustrator. It

I think you're right that there's a quantity of data where using D3.js will start to be more productive than doing it manually in something like Illustrator. It's an instance of a trade off that we're all familiar with: at what point is it worth building a tool instead of doing the task manually? It's also worth bearing in mind that the point where it makes sense to use a tool is reached more quickly with proficiency in the tool - if you have Mike Bostock working for you I imagine most visualisations will be easier to produce with D3.js! I think the other significant aspect is the speed of iteration. In my experience, producing good visualisations takes lots of iteration and those can take a lot of time if you have to alter each data item by hand. I've often reverted to paper and sketched out a revision that's way a few times before going back to the drawing program. I think it's likely that it's much quicker to iterate using D3.js (in the right hands).