A better version of this functionality will be present in core CiviCRM soon (hopefully by Nov 2019 - I'll update this readme then).
In the mean time you can install from the dc branch (download extension .zip) to bring charts back to your CiviCRM. You'll want/need to uninstall this once the core update happens.
The master branch replaces core open flash chart with chartjs (from CDN).
This draws on a <canvas> which makes copying the image easy.
The dc branch replaces them with 'dc' drawn charts using the dc (and
crossfilter and d3) bundled with CiviCRM. This creates a <svg> chart
which means you get a resolution independent chart that you could (given
nice browser plugins/skills) edit in an external editor.
Note that the d3 and dc included in core are very old. They still work but they are notably outdated. The latest verion of Civisualize, for example, uses more up to date libraries.