Graphing without thinking

This website claim to be monitoring property prices area by area and produce graphs to show you how these prices are faring. Unfortunately they have a problem, looking at London NW3 for example it would seem that prices for detached houses and semi-detached houses have gone through the floor from around £3,000,000 to near zero. Oddly prices for terraced houses and flats have fared better. A more logical explanation would be that there have been no sales of detached houses and semi-detached houses in the recent past and so no data. Do MSN.local even look at the graphs their system is producing?