Here's a link to a visualization:

http://vizwiz.blogspot.com.au/2012/05/how-common-is-your-birthday-find-out.html

Unfortunately they don't show frequencies there, but just show the ranks. Obviously it amplifies the differences in a really good way, but at the same time makes it harder to even guess the possible causes.

I tried to google for relevant plots, but all I found was an even weirder claim that those born in winter are also (statistically speaking) born in poorer, unhappier and less educated families: http://andrewgelman.com/2009/09/who_has_babies/

So, why the pattern at all? Is there any biology in it? Or are people really that clever and efficient that they can choose when to conceive, in order to give birth in a certain month? And why this month? What's the logic behind all this data?