track oa-share over time

[maxheld83]

trafficstars

the development of the share of open access publications is here listed as one of the criteria for successful transformative agreeements: https://www.coalition-s.org/addendum-to-the-coalition-s-guidance-on-the-implementation-of-plan-s/

[maxheld83]

this might depend on / be easier or more reliably in conjunction with #49