How is the data collected?!
While Duolingo provides some data through their API, it doesn't contain all the information you need to create the graphs below. However, it does contain information about the total XP in each in language that you are learning. Hence, I created a Python program that fetches this information and puts it in to a Google spreadsheet everyday. Because of this we are able to track the XP in each language over time.
Language stats and stuff
Here are a list of some interesting graphs that I have created from the data that I have collected from Duolingo. I only started collecting data automatically in December, and the data collected between August and October was collected manually. There is a lot of data missing between October and December.