I read it like 10 times and honestly I do not understand what kind of chart you have in mind. To compare total amount of posts in local boards vs general board as a whole or local boards vs all other boards? Or something completely different?
Sorry for my broken English.
My idea is to assess the level of activity in the local boards.
So, the first idea to determine how lively the “local boards ecosystem is”, would be to calculate the ratio between the number of posts in the local boards, as a whole, to the number of posts in the forum.
The second indicator would be the same ratio, but considering merits. So it would be merits in all the local boards over the merits across the whole forum.
Hope this clarifies.