I've been studying this problem for a couple of days now without success ...
I've configured and deployed Graphite (with carbon and whisper) using the Puppet Graphite module on a CentOS 6.5 server. The webapp will only render text in the graphs if I select Courier as the font face.
I know that Graphite uses cairo to render the PNG graphs, and that relies on freetype or the fontconfig library. All of those are installed.
Other people have solved this problem by installing the "bitmap-fonts" package. The package installed on my server is "bitmaps-fonts-compat" which one would hope is compatible with "bitmap-fonts".
The version of cairo provided in the RedHat repos is 1.8 (a few revisions old).