efficiency calculation of dc-dc converter
The curve shown in the graph is meant to be a typical performance chart, and it represents the overall input-to-output efficiency for the specific circuit shown. It includes all of the losses, including the MOSFET inside the chip, the external diode, and other miscellaneous losses.

It's just meant to give you a general idea of what kind of efficiency you can expect from a circuit built using the chip. They generally come up with a circuit that shows the chip in its best-case scenario, so you can expect that any other circuit you build can only do as well or worse, but never better.