When graphing the results of a factorial experiment with two independent variables, what determines which variable is placed on the x-axis and which is represented by formatting features like bar color or line style?

A researcher conducts a 2 × 3 factorial experiment examining the effects of therapy type (cognitive-behavioral vs. psychodynamic) and session frequency (weekly, biweekly, monthly) on anxiety scores. When creating a bar graph of the results, the researcher places session frequency on the x-axis and uses different bar colors to represent therapy type. A colleague argues that the graph is incorrect because the variable with fewer levels must always go on the x-axis. The colleague's criticism is valid.