The trouble with this graph is that it assumes the buses/trains are full. It obviously doesn't apply to a bus with one passenger on it, which takes up even more space than a car.
From which we can observe the problem. Alice is at home and wants to go to work. She travels along a low density street for a few miles, enters a congested thoroughfare for a few more miles, then travels along another low density street to her destination.
She can't take a bus at the endpoints because she would be the only one on it, but if she took a bus only on the congested thoroughfare then she'd end up on the far side of it without a vehicle to get the rest of the way to her destination.
To fix this the thing you need before mass transit (by several years) is zoning rules that allow higher density construction in a higher percentage of the land area where people live and work.