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 $x=3$ to $x=6$ ? Express your answer as a fraction or decimal.