That plot is indeed interesting, but it's important to keep in mind that it only shows the distributions of optional payments that were actually made. It doesn'

That plot is indeed interesting, but it's important to keep in mind that it only shows the distributions of optional payments that were actually made. It doesn't take account of how many people downloaded and played the game without paying anything (it's a free game after all). Nor does it give an indication of the total payments received. I'd really like to see some headline figures comparing the revenues for free games vs paid games (ideally a graph of price/revenue for paid games) rather than just a comparison of the average 'tip' for each category. It seems pretty intuitive that, if one is inclined to leave a tip, it will be higher if the game was initially free.