def _vector_field(q, dotx_func, doty_func, x, y, Is):
    """
    Add the vector field of the x and y derivatives in phase space.

    Parameters
    ----------
    q : :class:`matplotlib.quiver.Quiver`
        Phase space quiver object.
    dotx_func : function
        User provided function giving :math:`\\dot{x} = f(x, y, Is(t))`.
    doty_func : function
        User provided function giving :math:`\\dot{y} = g(x, y, Is(t))`.
    x : :class:`numpy.ndarray`.
    y : :class:`numpy.ndarray`.
    Is : float
        Current (time dependent data).
    """
    q.set_UVC(dotx_func(x, y, Is), doty_func(x, y, Is))
