The following graphs give information about the fraction of horde and alliance players in PvP over rating. There are two graphs for 2v2, 3v3, 5v5 and RBGs each.
The first graph depicts the percentage of each faction depending on rating in each bracket.
The second graph shows the evolution of these percentages over time for three ratings: 1500 and above, 1800 and above as well as 2200 and above.
All those graphs are displayed separately for regions US and EU. Sorry for the mess ;o)