You're right, it does not appear clearly on this page. Only on the homepage. However the prices are in € and the per-city dashboard says clearly "We analysed thousands of flats all around Europe". Also the introducing video lives no room for doubt: https://www.youtube.com/watch?v=_a7g69kXn_o
From the homepage "This project is a personnal experimentation of how we can use a data-driven tools like D3.js to discover a Philosophy piece like the Tractatus logico-philosophicus (Logisch-Philosophische Abhandlung in its original name).
This book published in 1921 was written by Ludwig Wittgenstein. Its numbered propositions follows a tree structure, (1, then 1.1, 1.2 etc.) which made me think we could easily visualize it. Please note that this project is not an attempt to explain nor justify the book, I prefer to let you do that."