Pull/merge request from GitHUb

fix for issue #28 (closed) "[Visualization] problem with BG-20, BG-21, BG-27 and BG-28"

This one is from GitHub https://github.com/itplr-kosit/xrechnung-visualization/pull/1

in case it is a good pull/merge request, we have to do a triangle merge on CLI

Documentation of the merge from/to github:

• git remote add visufix https://github.com/itplr-kosit/xrechnung-visualization.git
• (git fetch visufix --tags)
• git fetch visufix pull/1/head:pr-1 (pr-1 stands for pull request 1)
• git push --set-upstream origin pr-1