graphics(
    surface(x*y, (x, -5, 5), (y, -5, 5), use_cm=True),
    surface(-x*y, (x, -5, 5), (y, -5, 5), use_cm=True))
# Expected:
## Plot object containing:
## [0]: cartesian surface: x*y for x over (-5.0, 5.0) and y over (-5.0, 5.0)
## [1]: cartesian surface: -x*y for x over (-5.0, 5.0) and y over (-5.0, 5.0)