def plot_histograms(ax, prng, nb_samples=10000):
    """Plot 4 histograms and a text annotation."""
    params = ((10, 10), (4, 12), (50, 12), (6, 55))
    for a, b in params:
        values = prng.beta(a, b, size=nb_samples)
        ax.hist(values, histtype="stepfilled", bins=30, alpha=0.8, density=True)
    # Add a small annotation.
    ax.annotate(
        "Annotation",
        xy=(0.25, 4.25),
        xytext=(0.9, 0.9),
        textcoords=ax.transAxes,
        va="top",
        ha="right",
        bbox=dict(boxstyle="round", alpha=0.2),
        arrowprops=dict(
            arrowstyle="->", connectionstyle="angle,angleA=-95,angleB=35,rad=10"
        ),
    )
    return ax