Mark's response is kind of bogus: it's true that Fedora shouldn't be lumped with RHEL unless you lump Debian with Ubuntu, but you can't say this for CentOS. It's literally RHEL with all the branding removed (and no support from RedHat).
However, the graph does point to a decline in the willingness to pay lots of money for a server Linux distro, which is an interesting trend.
If it's maketshare I assume the graph is in % and it could mean that there is a lot more servers being added and not a decline in installs of RHEL. I can see that with Amazon EC2 instances and other cloud platform Ubuntu would be really popular on those. Buying a RedHat license for you VPS instances might not be worth it but then CentOS vs Ubuntu would be more interesting there.
Is it just a question of paying lots of money for a distro? Or is it a question of the market decentralizing, and so vendor-centric solutions doing increasingly poorly?
BTW, this back and forth proves what Heisenberg forcefully argued in his book, "Physics and Philosophy," that theorizing fundamentally involves projecting your own assumptions onto data you see. We all see the same graph. We get to argue to no end as to what it means.
However, the graph does point to a decline in the willingness to pay lots of money for a server Linux distro, which is an interesting trend.