Over the next two weeks, collectors will spend millions at the three big auction houses in New York at Impressionist-modern and postwar-contemporary sales. The competition among bidders is fierce, but so is it among auction houses, which must fight to sell the finest works, offering juicy guarantees and aggressive estimates during this buoyant market, in which prices for the choicest art have soared. In the graph above, a look at how the number of ultra-high-priced lots, estimated at or above $5 million, has grown dramatically over the years in the November contemporary sales in New York. (Note that the graphs include special evening sales that include some lots that date from before the postwar period.)