Where to find statistics on the usage of JSON in web API's compared to XML [on hold]

I am currently working on my thesis which includes developing an API. One of the requirements of my thesis includes that it has to be in JSON format. Now I have to prove that JSON is more used in API's than XML, using statistics or scientific facts. I stumbled upon a few graphs taken from programmableweb.com, which are really helpful but are published in 2013 (example: http://ift.tt/2dw3ASr ). Now when I use the website and filter the API directory on the format, it only shows 10 apis for JSON and 4 for XML.

I also read a couple of blog posts published by different people who claim that JSON will be the de facto standard, but unfortunately no facts to back this up.

Does someone know an up to date source for finding up-to-date statistics on JSON and XML usage in the web API's?