def plot_swarm_2d(swarm, ax, **kwargs):
    """
    Plot an scatter of the particle position for a defined time step

    Parameter
    ---------
    swarm : :class:`pandas.DataFrame`
        DataFrame containing the particles positions for a specific time step. The
        positions of the particles are given by ``x``, ``y`` and ``z`` in meters. The
        ``cc0`` is the number assigned to each particle belonging to a finite element.
        The ``time`` is given in Ma. The index of the :class:`pandas.DataFrame`
        correspond to the step number.
    ax : :class:`matplotlib:Axes`
        Axe where the plot will be added.
    kwargs : dict
        Keyword arguments passed to :func:`matplotlib.pyplot.scatter`. By default the
        marker size ``s`` , the ``color`` and the alpha blending value ``alpha`` will
        be 0.2, 'black' and 0.3 respectively.
    """
    # Fix some parameter un the kwargs
    if "s" not in kwargs:
        kwargs["s"] = 0.2
    if "color" not in kwargs:
        kwargs["color"] = "black"
    if "alpha" not in kwargs:
        kwargs["alpha"] = 0.3
    # Plot
    return ax.scatter(swarm.x, swarm.z, **kwargs)