Elistan

One of the curves tops out at 210, the other at 130. By scaling the curves so that the both top on at the top of the chart gives maximum resolution to the changes in the readings. If the torque were scaled down to the same scale as horsepower, you wouldn't get as much resolution in the graph.

Imagine a graph of an F1 engine - if power and torque were on the same scale, the torque curve would be scrunched in the very bottom of the graph, and you'd have a difficult time making out any dips and spikes in it.