With elegance, wit, and precision, the septuagenarian Marian McPartland has made so much good music--which throughout the 70s and 80s traced a steadily progressing arc of creative growth--that it's easy to take her for granted. It became even easier once she added "oral historian" to her resume, hosting the popular and acclaimed public-radio interview series "Piano Jazz". Thanks to her versatility and ingenuity, McPartland inherited without challenge the crown of First Lady of Jazz when Ella Fitzgerald retired from performing a few years ago; Fitzgerald's passing last weekend only underscored McPartland's remarkable longevity as a creative artist. The first British jazz musician to emigrate to the U.S. (in 1946, beating out George Shearing by a year), McPartland started out playing the Louis Armstrong-influenced traditional-jazz style of her husband, the famous Chicago cornetist Jimmy McPartland. But within a few years she had proved herself more than adaptable to both bebop and the cool modernism of Lennie Tristano. McPartland really blossomed from the 70s on, however, when each new recording seemed to reveal another stylistic twist and the work of yet another young jazz composer. She combines her rhythmic strength with a rose-petal touch that recalls the music of her contemporary Hank Jones; the graceful filigrees of her style, forged in the swing era, can't obscure the flexible steel at the core of her music. Her Chicago trio will include Jim Cox on bass and Charles Braughan on drums, neither of the strangers to either McPartland or those who appreciate the highest musical craftsmanship. Friday and Saturday, 9 and 11 PM, and Sunday, 4 and 8 PM, Jazz Showcase, 59 W. Grand; 670-2473.