I added some legend where I saw it fit + a link to wikipedia explaining what a population pyramid is. That said, most people that land on the page are looking for population pyramids (from their google keywords) so there is not so much point to explain what it is.
Also, in a tablet, you can point at the chart with your finger and get the same information. Or, in this case, the lack of information. :)
EDIT: I just tried this chart on my tablet and you get the same data by pointing.