Understanding how quantities in multivariable relationships covary is crucial for studying thermodynamics. To study how students reason about multivariable covariation, we asked junior-level physics majors to consider an ideal gas and the corresponding contour graph of temperature vs. volume and pressure. We asked these students to identify how the temperature changes (a) when the volume changes and (b) when the pressure changes, without specifying how the other variable changes. We found that students used three kinds of reasoning: (1) referencing the features of the provided graph, (2) using physical knowledge about the gas, and (3) specifying a change in the third variable. Almost all students, regardless of the type of reasoning they used, answered that the temperature could only change in one way. These results suggest that instruction focusing on the different ways thermodynamic variables can change together might help students develop more sophisticated covariational reasoning skills.