The d3 images on the streaming-concepts.html page are not appearing

When I was reading this page it was referring to colors and streams and I could not see any images. It turns out that this error is being thrown when browsing to the page and looking at a javascript console:

Mixed Content: The page at 'https://edgent.apache.org/docs/streaming-concepts.html' was loaded over HTTPS, but requested an insecure script 'http://d3js.org/d3.v3.min.js'. This request has been blocked; the content must be served over HTTPS.

This seems to be due to attempting to fetch the d3 library at a non-https url. There is this post about this:

https://github.com/jsbin/jsbin/issues/2549

Also, I tried changing the url from 'http://d3js.org/d3.v3.min.js' in my local incubator-edgent-website 'streaming-concepts.html' to 'https://d3js.org/d3.v3.min.js' and it seems to work fine. I.e, the images appear.