plot3d(x*y, -x*y, (x, -5, 5), (y, -5, 5), use_cm=True)
# Expected:
## Plot object containing:
## [0]: cartesian surface: x*y for x over (-5, 5) and y over (-5, 5)
## [1]: cartesian surface: -x*y for x over (-5, 5) and y over (-5, 5)