Using a typical performance chart involves putting the the Pressure Altitude and the Ambient temperature, both of which are readily available.
The curves of the chart combine these two variables in such a way as to account for the effecst of Density Altitude, which is not readily available.
If Density Altitude was readily available the performance charts would be desiged to enable us to put in Density Altitude directly. But Density Altitude is not so readily available, so the charts are designed to use the two variables ( Pressure Altitude and OAT) that are readily available.