Use Chart.js in Drupal page
You need to define it in either a module, or the libraries.yml file of your theme. Then you need to attach it to the page. See the "CDN / externally hosted libraries" of this doc page:

https://www.drupal.org/docs/8/creating-custom-modules/adding-stylesheets-css-and-javascript-js-to-a-drupal-8-module

Then you need to attach the library - this is fairly straightforward in mytheme.theme. 

Example mytheme.theme file:

function mytheme_page_attachments(array &amp;$attachments) {
  $attachments['#attached']['library'][] = 'mytheme/chartjs';
}