Data and accuracy
Where the data comes from
Pokémon, forms, abilities, base stats and sprites come from PokeAPI. Every cell of the current type chart is checked against PokeAPI each time the data is updated.
Evolutions
Evolution methods come from the evolution tables unpacked from each game, published in the open-source PKHeX project, and are compared with PokeAPI each time the data is updated. The tables don't name places, so those come from PokeAPI. Pokémon GO names and methods come from player guides, since Niantic doesn't publish the full list.
What the bars mean
The HP bars show relative damage from the type multiplier, with 4× emptying a bar. They are not a damage calculation: moves, stats, items and levels aren't included.
Counters
Counter suggestions compare types and base stats only. They ignore movesets, so a listed Pokémon may not learn the move you need in your game.
Unofficial
This is a free fan project and isn't affiliated with Nintendo, Game Freak, Creatures Inc. or The Pokémon Company.