def plot_tops(self, ax, legend=None, field=None, **kwargs):
        """
        Plotting, but only for tops (as opposed to intervals).
        """
        if field is None:
            raise StriplogError('You must provide a field to plot.')

        ys = [iv.top.z for iv in self]

        try:
            try:
                ts = [getattr(iv.primary, field) for iv in self]
            except:
                ts = [iv.data.get(field) for iv in self]
        except:
            raise StriplogError('Could not retrieve field.')

        for y, t in zip(ys, ts):
            ax.axhline(y, color='lightblue', lw=3, zorder=0)
            ax.text(0.1, y-max(ys)/200, t, ha='left')

        return ax