In 1975, Cy Twombly painted his friend, dealer Yvon Lambert, as a tall, tremulous, vertical blue line — imposing, elegant, elusive. In 2006, Dutch conceptualist Stanley Brouwn turned the image horizontal and declared a painted trestle table “Portrait of Yvon Lambert” — solid but wacky. Last year there appeared in the courtyard of the Lambert Collection in Avignon a cluster of ceramic pots planted with Californian seeds: Los Angeles artist David Horvitz’s “A Portrait (of Yvon Lambert)” — the gallerist as grounded, seminal, global.