in:Flux has two coordinate systems: one relating to the Global set of coordinates and another based on the Domain of the ventilation simulation.
For visuals such as contours and vectors, the coordinate system is based on the Domain of the corresponding ventilation. You may view the coordinate system axis and grid by selecting the Guide Visible checkbox in the properties panel of each ventilation simulation, shown in the figure below for a westerly wind.
Zooming in you will see the grid and labels at every 10 meters.
The Domain coordinate system is as follows for any simulation:
X-axis being the streamwise direction
Y-axis being the cross-stream direction
Z-axis being in the vertical direction
Thus, for a vertical contour along the center line of the domain (in the same direction as the flow), select the ZX Plane with a offset of Y = 0 as in the below figure.