There is no second game released for now to compare the growth of Splatoon. But Uncharted should definitely be there.

Third person shooters are most likely seeing a dip in overall popularity, but that's only because games like Gears of War are getting less sales and Resident Evil switched to first person with 7. In actuality, the third person genre is still pretty strong, Metal Gear Solid V I'm pretty sure is the best selling Metal Gear game including digital. Uncharted had it's most popular entry to date. The Tomb Raider series continues to sell extremely well(I'm honestly shocked at how well the PS4 version did). And Ghost Recon just saw a huge resurgence.

I don't know if i'd consider Horizon as a third person shooter, and Metal Gear is a bit iffy too ... but Horizon is extremely lucrative as well. I think we're mostly seeing third person multiplayer centric shooters being nudged out for singleplayer ones. Except Splatoon.

Now that I think about it, third person shooters might be more popular than ever.... weird, I thought they were on the decline! But Splatoon, Metal Gear Solid V, Tomb Raider Reboot, Uncharted 4, Horizon(maybe), all say otherwise. The only 3rd person franchise that got a big decline is Gears of War and SOCOM. 

Simple. They are all Action-Adventure games, not TPS. Also Last of Us is not even a franchise yet.

But shooting is a bigger element in those games than in Mass effect. And RE and MGS use shooting in a very different way than other third person shooters. Uncharted, Tomb Raider, and The Last of Us are all extremely third person shooting heavy, whereas Mass Effect 3 and MGS V and RE 5-6 are the only really shooting-oriented ones.

But MGS is not a Third Person Shooter, it's a Stealth Action game. It's literally right in the tilte of the game: Metal Gear Solid: Tactical Esponage Action.

It's always kind of hard to trace a line between a two different genres but from my point of view (and the one of wikipedia too) Uncharted and Tomb Raider are classified as Action-Adventure, not TPS. If I included them in my graph some other people would have complained.

Stealth is a subgenre. MGS is still part of the TPS genre.

No. If Uncharted is an Action game, then so is MGS. There's nothing about MGS that makes it more of a shooter than Uncharted.

Even Wikipedia calls MGS an action-adventure game. https://en.wikipedia.org/wiki/Metal_Gear_Solid

Uncharted on the otherhand...

"Uncharted is an action-adventure third-person shooter platform video game series"

I wonder how steep the growth for Splatoon 2 will be

Actually, they are putting both franchises in both the 'third-person shooter' and 'Action-Adventure' category. I have just not included Uncharted into the FPS genre as the game's action is less orientend on shooting people than MGS.