Yes, you're reading it wrong. Every vertical slice of that graph is the market share % at that point in time. What happened in that period is that Symbian grew

Yes, you're reading it wrong. Every vertical slice of that graph is the market share % at that point in time. What happened in that period is that Symbian grew in market share over that period, and by chance, Symbian is at the top of that graph, so it looks at first glance that things dipped. If Symbian had been at the bottom of that graph, representing the exact same data, you'd have thought everything went up at that point. Imagine this, the graph is perfectly valid and representing the same data if you flip it upside down. It's all about their size relative to eachother, not their vertical position.