Fix bug that prevents loading of charts without series

Created originally on Bitbucket by Eiríkur Torfason

Was already merged in Bitbucket before import, marked as merged by the import user

Fixes issue #1250 and adds a test to verify the fix.

I’ve run the tox tests locally with interpreters for 3.6, 3.7 and 3.8.

I’m new to Bitbucket. I’m not certain that I was successful in enabling pipelines for the forked repo…