Shade across x values where boolean mask is `True`

    Args
    ----
    ax: pyplot.ax
        Axes object to plot with a shaded region
    ind: ndarray
        The indices to use for the x-axis values of the data
    mask: ndarray
        Boolean mask array to determine which regions should be shaded
    facecolor: matplotlib color
        Color of the shaded area

    Returns
    -------
    ax: pyplot.ax
        Axes object with the shaded region added

def plot_shade_mask(ax, ind, mask, facecolor='gray', alpha=0.5):
    '''Shade across x values where boolean mask is `True`

    Args
    ----
    ax: pyplot.ax
        Axes object to plot with a shaded region
    ind: ndarray
        The indices to use for the x-axis values of the data
    mask: ndarray
        Boolean mask array to determine which regions should be shaded
    facecolor: matplotlib color
        Color of the shaded area

    Returns
    -------
    ax: pyplot.ax
        Axes object with the shaded region added
    '''
    ymin, ymax = ax.get_ylim()
    ax.fill_between(ind, ymin, ymax, where=mask,
                    facecolor=facecolor, alpha=alpha)
    return ax