Hello :) I'm fairly new to DNN and I'm currently on version 07.04.00 (353).

I want to use some d3.js plots on the site, but it doesn't seem to work. I've tried it on some other sites, that doesn't use DNN, where it has worked. I don't know if it is the problem, but on the other sites, when I'm using the inspector tool in chrome a svg with the plots is shown - but not when copied into DNN. I've made an id where the script is supposed to run and put the script in the header of the modal, but it doesn't seem to work. I've put the d3.js script required for the specific script to work in the header of the page.

Any input on this matter will be welcomed! 
Thanks in advance. 

If your site is a secure site, you have to make sure all the source of link and images or other related files are in https as well.
One trick you can do is by omitting the http: and only use the double forward slash, like below example:
//ajax.googleapis.com/ajax/libs/ext-core/3.1.0/ext-core.js