No charts graphics, neither with oder without Highcharts API

Looks a bit like the following issue:
https://stackoverflow.com/questions/48238105/highcharts-renders-the-chart-as-solid-black-and-missing-fill-attribute-on-rec

Seems like newer versions of highcharts moved styling to a separate css file. Please try the suggestion in the stackoverflow topic.

Or see if you can download a 4.x version somewhere.

OK, thank you!
I added the HighCharts CSS to my CustomCSS and it worked.
Greez!

I downloaded the v6 of Highcharts to test. They have a ‘classic’ file with styling included in the JS file. That file is in the /code folder.
And they have a styled mode version without styling in the JS file.

quote from their readme file:

This directory contains generated source code.

WHAT FILES TO USE
- The files for classic mode are available in the /code folder. In this mode, 
  styles are set as options in the chart configuration.
- The JavaScript files for styled mode are available under the /code/js/ folder.
  In these files, most presentational code is removed, so without styling it
  will draw an ugly black-and-white chart if anything.
- The default CSS file for styled mode is available as /code/css/highcharts.css.
  This CSS is in turn based on a SCSS file, highcharts.scss that can be found
  in the Highcharts GitHub repo.