Since Joomla! 3.5 a statistics plugin will submit anonymous data to the Joomla Project. This will only submit the Joomla version, PHP version, database engine and version, and server operating system. The graphs marked as "recent" shows only the stats for the last 180 days.
This data is collected to ensure that future versions of Joomla can take advantage of the latest database and PHP features without effecting significant numbers of users. The need for this became clear when a minimum of PHP 5.3.10 was required when Joomla! 3.3 implemented the more secure Bcrypt passwords.
We are making this data publicly available through our public API but here we will be providing graphs representing the Joomla version, PHP versions and database engines of sites in the interest of transparency to the community and to help 3rd party developers.