problems with diagram Bgee/014

[VladimirAlexiev]

https://sib-swiss.github.io/sparql-examples/examples/Bgee/014.html:

• the big "optional" box misleads that all the props are optional since their labels appear in the box.
  • in fact only ?uniprot in that box is optional: the other variables are outside of the box
• it tried to fit all labels inside the box, which made the box unnecessarily wide.
• For some reason it twisted all arrows to the right, rather than drawing them straight downwards

I don't have much experience with Mermaid, but maybe there are some ways to tweak the diagram?

Eg in PlantUML one can specify arrow lengths, see this example (note to self: uses <style> instead of skinparam)