It's the average modern rental cost per month of a one bedroom flat (I.e. Two-room) at each stop on the London tube map.
I came up from rural Cornwall to go to university in Uxbridge, top left. When I was single I moved to Boston Manor (bottom left) to work in South Kensington, then my first married home was a one bedroom flat in Richmond (bottom left) .
We moved out to Kent to buy and because they had grammar schools. I couldn't afford to send my children to public school and didn't want to ruin their prospects by dumping them in an inner city comprehensive.
The map is high res of you tap on it.