It really all depends on the kind of visualization you're hoping to do.  As far as interfacing with a database, check out http://processing.org/reference/libraries/index.html#data_protocols, there is a MySQL library, so the database connection should be  fairly easy.  However, any sort of visualization involving millions of points is going to be quite processor intensive - you're not going to be able to plot a million points per frame in Java (at least on any normal computer), no matter how fast the libraries you use are!

Think about what exactly you need to display, and see if you can figure out a way to build it up incrementally without having to keep all the data around or process it all for each frame - I have used Processing to visualize millions of pieces of stock data before (I was calculating distributions and testing options valuation techniques based on the whole history of available daily data for every currently listed stock), so it's definitely possible, but you have to choose your algorithms very carefully with that much data.