During a study, the temperature, in degrees Celsius (${}^{\circ }$C), of the air in a chamber was recorded to the nearest integer at certain times. The scatterplot shows the recorded temperature $y$, in ${}^{\circ }$C, of the air in the chamber $x$ minutes after the start of the study.

What was the average rate of change, in ${}^{\circ }$C per minute, of the recorded temperature of the air in the chamber from $x=4$ to $x=6$?