x,y=var('x y')
plot_vector_field((y,-x), (x,-5,5), (y,-5,5),aspect_ratio=1)
plot_vector_field((cos(x*y),sin(x*y)), (x,0,5), (y,0,5),aspect_ratio=1)
plot_vector_field((cos(x*y),sin(x*y)), (x,3,4), (y,3,4),aspect_ratio=1)
plot_vector_field((4*x**3-8*x,4*y**3-8*y), (x,-5,5), (y,-5,5),aspect_ratio=1)
plot_vector_field((4*x**3-8*x,4*y**3-8*y), (x,-2,2), (y,-2,2),aspect_ratio=1)
plot_vector_field((y,x), (x,-5,5), (y,-5,5),aspect_ratio=1)
plot_vector_field((-y,x), (x,-5,5), (y,-5,5),aspect_ratio=1)
plot_vector_field((x,y), (x,-5,5), (y,-5,5),aspect_ratio=1)
plot_vector_field((-x,-y), (x,-5,5), (y,-5,5),aspect_ratio=1)

x,y,z=var('x y z')
plot_vector_field3d((y,z,x), (x,-2,2), (y,-2,2), (z,-2,2),plot_points=[7,7,7])
plot_vector_field3d((-1,-1,-1), (x,-2,2), (y,-2,2), (z,-2,2),plot_points=[7,7,7])

plot_vector_field3d((x*y,y*z,z*x), (x,-1,1), (y,-1,1), (z,-1,1),plot_points=[9,9,9])
plot_vector_field3d((-y,-z,-x), (x,-1,1), (y,-1,1), (z,-1,1),plot_points=[9,9,9])