This is rubbish. Loads of people have equity. This only takes into account first time buyers.
I would imaging that first time buyers are in the minority, compared to people buying their second, third or fourth homes- with resultant equity following them.
I am on my fourth consecutive house purchase, this chart is meaningless for lots of people.
As is the median prices as pootles points out. The average house price in my village is around £200K, but there are lots of properties around the £400K mark, equally, lots at £135K, even a large number of ex council stock which are snapped up by first time buyers, a 2 bed semi for £95K.
I think someone at the Guardian has a nice new graphics package he is keen to show off.