There is something seriously wrong with this chart.
Toronto does NOT have higher urban density than New York City (or London). Moscow is NOT denser than Tokyo.
The numbers are a little bit old (“West Berlin”), so that may explain some of the unusual positions. The rest is probably because these things are usually calculated for city-regions. Although Toronto is less dense than New York City, the Toronto region is certainly more dense than the New York region or any other American region because we sprawl less exuberantly. New York City is famously (among planners) less dense than Los Angeles if you count their respective suburbs because Los Angeles is hemmed in by the mountains and New York City sort of trickles out into the countryside.
I like how the Australian cities just line up like that, almost like they planned for a linear relation.
Also, doesn’t it make sense that the more dense a city is, the less transport-related energy would be consumed? The way I’m imagining it, the more dense a city is, the less distance people will have to travel and the more likely there will be an efficient mass-transit system (or at least the potential of one).