"Lady Day at Emerson's Bar & Grill" gives a balanced musical and dramatic portrayal of jazz legend Billie Holiday late in her career. Playhouse on Park's production of Lanie Robertson's well-known play, which has been around since the 1980s, stars Danielle Herbert as Holiday and Nygel D. Robinson as her longsuffering accompanist and is directed by Stephanie Pope Lofgren. It runs through Oct. 16.