Plots the distribution of events in time.

    Parameters
    ----------
    events : pd.Series
        A pd.Series whose index contains at least 'date' level.
    num_bars : integer, optional
        Number of bars to plot
    ax : matplotlib.Axes, optional
        Axes upon which to plot.

    Returns
    -------
    ax : matplotlib.Axes

def plot_events_distribution(events, num_bars=50, ax=None):
    """
    Plots the distribution of events in time.

    Parameters
    ----------
    events : pd.Series
        A pd.Series whose index contains at least 'date' level.
    num_bars : integer, optional
        Number of bars to plot
    ax : matplotlib.Axes, optional
        Axes upon which to plot.

    Returns
    -------
    ax : matplotlib.Axes
    """

    if ax is None:
        f, ax = plt.subplots(1, 1, figsize=(18, 6))

    start = events.index.get_level_values('date').min()
    end = events.index.get_level_values('date').max()
    group_interval = (end - start) / num_bars
    grouper = pd.Grouper(level='date', freq=group_interval)
    events.groupby(grouper).count().plot(kind="bar", grid=False, ax=ax)
    ax.set(ylabel='Number of events',
           title='Distribution of events in time',
           xlabel='Date')

    return ax