Art World Expansion Erodes Sotheby's/Christie's Marketshare

Felix Salmon asked Artnet for some data on global market share for Christie’s & Sotheby’s. They gave him this chart which shows, despite steady sales growth, evaporating marketshare for the leading auction houses:

The chart shows the growth of other auction houses. China is an obvious place where auction houses are expanding but it’s not the only one. Around the world, and around the US, regional auction houses seem to be expanding their businesses. Then there’s the online businesses like Saffronart and Artnet. It isn’t clear whether the chart reflects the growing sales at other outlets like 20 x 200.