visualisation: improvements before publishing

[kdutia]

• [x] clean up legend (add an overflow signifier)
• [x] replace 'this is a demo' text with something more useful
• [x] add twitter card etc

extra (if easy):

• [ ] ~colour type indicator in tooltip the same as the dot colour~ requires rewriting of the library which creates the tooltips
• [ ] ~can we use size to encode number of connections?~ whilst we can give dots different sizes, as they are not circular and their boundaries aren't clear it then becomes difficult to distinguish between a large dot vs lots of dots