This recent S&P 500 addition is now the best-performing stock in the index

A newcomer to America's most-watched index is now its top-performing stock of the year. Robinhood Markets  (HOOD) , which joined the index on Sept. 22, is now the index's top year-to-date performer. Up 219% this year, the brokerage app now sits atop the S&P 500's year-to-date performers, surpassing Seagate Technology  (STX,)  which recently took the throne from one-time index leader Palantir Technologies  (PLTR) . Those two firms are up 164% and 142.7% year-to-date respectively; now the second and fourth- ...