The y-axis measures "momentum: average growth of US patents in cluster, 1997-2006." Say the authors:
Innovation clusters around the world can be classified based on their growth and diversity dynamics: 'hot springs' are small, fast-growing hubs on track to become world players; 'dynamic oceans' consist of large and vibrant ecosystems with continuous creation and destruction of new businesses; 'silent lakes' are older, slower-growing hubs with a narrow range of large established companies; 'shrinking pools' have been unable, so far, to expand beyond their start-up core and so find themselves slowly migrating down the value chain.The concept here is interesting enough, though I'm not sure what the point of going to all the trouble of presenting this much data is if you're not going to label most of it. There are a bunch of cities that are classified as shrinking pools? Well, bully. Now which ones are they? The graphic doesn't say. Anyways, you can read the article at What Matters for details on their methodology.