Auxiliary function to plot violinplots.

def _violinplot(val, shade, bw, ax, **kwargs_shade):
    """Auxiliary function to plot violinplots."""
    density, low_b, up_b = _fast_kde(val, bw=bw)
    x = np.linspace(low_b, up_b, len(density))

    x = np.concatenate([x, x[::-1]])
    density = np.concatenate([-density, density[::-1]])

    ax.fill_betweenx(x, density, alpha=shade, lw=0, **kwargs_shade)