Graffiti Artist Alec Monopoly -- Makes Burger Dream Come True ... WITH a Rolls!

Renegade street artist Alec Monopoly's bad boy rep just took a serious hit ... 'cause he did something really nice ... involving a sick kid, a sweet whip and legendary meat.

Monopoly hit the town with 19-year-old cancer survivor Zachary Elliott ... first, picking him up at LAX in a white Rolls-Royce Phantom ... then whisking him off to the nearest In-N-Out Burger for his first Double-Double.

Elliott -- who's from Greenville, SC -- lost a leg to bone cancer and says Monopoly's art inspired him while he was fighting for survival ... so the Children's Wish Foundation hooked them up.

Monopoly also showed off his skills ... taking Elliott to a graffiti project in Santa Monica, where he covered a wall with his signature piece ... the rich dude from Monopoly.