scalar_expr = sqrt((-sin(y))**2 + cos(x)**2)
plot_vector([-sin(y), cos(x)], [2 * y, x], (x, -5, 5), (y, -3, 3),
    n=20, legend=True, grid=False, xlabel="x", ylabel="y",
    scalar=[scalar_expr, "$%s$" % latex(scalar_expr)], use_cm=False,
    rendering_kw=[
        {"cmap": "summer"}, # to the contour
        {"color": "k"},     # to the first quiver
        {"color": "r"}      # to the second quiver
])
# Expected:
## Plot object containing:
## [0]: contour: sqrt(sin(y)**2 + cos(x)**2) for x over (-5.0, 5.0) and y over (-3.0, 3.0)
## [1]: 2D vector series: [-sin(y), cos(x)] over (x, -5.0, 5.0), (y, -3.0, 3.0)
## [2]: 2D vector series: [2*y, x] over (x, -5.0, 5.0), (y, -3.0, 3.0)