Including other JavaScript libraries in custom visual

Could someone please explain how this is done? I want to use the d3 library but I don't know how to actually use it from within my visual.ts file. I've installed it with npm and added it to the externalJS part of pbiviz.json, is there anything else I need to configure before I can include and use it in visual.ts?

When I add 

import * as d3 from "../node_modules/d3";

to the top of visual.ts then I get the following error:

I've also tried just 

import * as d3 from "d3";

to no avail.

I've looked through the source of the example custom visual but couldn't actually find any kind of importing/including... In barchart.ts they just start using d3 straight away, which is a bit confusing. When I try that I get:

Could someone please show me what I'm doing wrong?