v = [sin(x - y), cos(x + y)]
plot_vector(v, (x, -3, 3), (y, -3, 3),
    quiver_kw=dict(color="black", scale=30, headwidth=5),
    contour_kw={"cmap": "Blues_r", "levels": 15},
    grid=False, xlabel="x", ylabel="y")
# Expected:
## Plot object containing:
## [0]: contour: sqrt(sin(x - y)**2 + cos(x + y)**2) for x over (-3.0, 3.0) and y over (-3.0, 3.0)
## [1]: 2D vector series: [sin(x - y), cos(x + y)] over (x, -3.0, 3.0), (y, -3.0, 3.0)