How to push values from views attachment to Javascript?
It really depends on what you want and what data structure you have.
Normally I would say if you want only chart on the page/views, than create a new views type (Like block, page, etc) and then you can do whatever you want, and you can still attach it to another views. (I think this is it: https://www.drupal.org/docs/8/creating-custom-modules/building-a-views-display-style-plugin-for-drupal-8)
If you just want to show a chart side-by-side with some other data, maybe you need to create a new field (for views), than you can put there the stuff. (https://www.webomelette.com/creating-custom-views-field-drupal-8)
You can also hook in any time to a view, but it will make a mess after a while, however, every time you need to use the drupalSettings (Pass variable to drupalSettings) if you want to pass php data to javascript and some custom JS code to glue the ChartJS to the drupalSettings.