The activity on 2.4 has been drowned out on those graphs by the madness of commits that were necessary to get 2.3 going with the new GUI. 2.3 was a massive undertaking on the frontend of things, 2.4 has been more backend work and cleanup.
If you look at https://redmine.pfsense.org/projects/pfsense/activity there is plenty of activity every day (and that doesn't include the ports or src repos).
Plus as we near 2.4-RELEASE things slow down in some areas as we hit a wall of harder bugs that take more time to fix but result in fewer rapid commits.
A good chunk of the tickets assigned to 2.4 are in a feedback state waiting for people to respond and confirm the bugs are fixed. As usual some tickets assigned to 2.4 will be pushed forward if they are not going to make it into 2.4 as well. And undoubtedly there are some bugs in the 'new' state that have been addressed or are otherwise invalid but haven't been caught and closed yet.
If you want to see the percentage of tickets closed on 2.4 rise, go look at tickets in the feedback state and test them, and respond on the tickets.