Refactor bubble-chart-legend

[roschaefer]

After discovering how to import 3rd party libraries for d3 and installing d3-svg-legend we definitely should refactor bubble-chart-legend component to use that plugin.

[mattwr18]

@roschaefer says: for later reference

it would be great to refactor the chart next to the bubble chart visualization

#786
this whole component:
https://github.com/roschaefer/rundfunk-mitbestimmen/blob/master/frontend/app/components/visualize/bubble-chart-legend.js
can be replaced with sth similar to:
https://github.com/roschaefer/rundfunk-mitbestimmen/blob/master/frontend/app/components/visualize/graph/graph-visualization.js#L48

[mattwr18]

for creating seed data that is from more than three months in the past:
https://en.wikipedia.org/wiki/Slowly_changing_dimension#Type_4
Also, have a look at these:
https://github.com/ifad/chronomodel
https://github.com/ifad/chronomodel/blob/master/spec/support/helpers.rb
tests using time travel:

rundfunk-mitbestimmen/backend/spec/requests/statistic/broadcast_spec.rb

Line 212 in 0dbbd30

data.each do |d|

[mattwr18]

This is the link to the d3-legend from susie lu with examples:
https://d3-legend.susielu.com/