I am probably reading the chart a little wrong, but I am curious about that slump in between January 2005 and January 2006 for all mobile OSes (or maybe all but Symbian).
Related: it's important to realize when reading this chart that the absolute number of phone shipped in this period was exploding from year to year: it's quite possible that every single manufacturer on the chart (except maybe Palm) shipped more devices every single year; Android and Apple just grew faster than the others.
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.
2005 was when the first Nokia N series was released.