Accumulate the legend handles and labels for all subplots
        and set up the legend

def _adjust_legend(self, overlay, axis):
        """
        Accumulate the legend handles and labels for all subplots
        and set up the legend
        """
        legend_data = []
        dimensions = overlay.kdims
        title = ', '.join([d.name for d in dimensions])
        for key, subplot in self.subplots.items():
            element = overlay.data.get(key, False)
            if not subplot.show_legend or not element: continue
            title = ', '.join([d.name for d in dimensions])
            handle = subplot.traverse(lambda p: p.handles['artist'],
                                      [lambda p: 'artist' in p.handles])
            if isinstance(overlay, NdOverlay):
                key = (dim.pprint_value(k) for k, dim in zip(key, dimensions))
                label = ','.join([str(k) + dim.unit if dim.unit else str(k) for dim, k in
                                  zip(dimensions, key)])
                if handle:
                    legend_data.append((handle, label))
            else:
                if isinstance(subplot, OverlayPlot):
                    legend_data += subplot.handles.get('legend_data', {}).items()
                elif element.label and handle:
                    legend_data.append((handle, element.label))
        all_handles, all_labels = list(zip(*legend_data)) if legend_data else ([], [])
        data = OrderedDict()
        used_labels = []
        for handle, label in zip(all_handles, all_labels):
            # Ensure that artists with multiple handles are supported
            if isinstance(handle, list): handle = tuple(handle)
            if handle and (handle not in data) and label and label not in used_labels:
                data[handle] = label
                used_labels.append(label)
        if (not len(set(data.values())) > 0) or not self.show_legend:
            legend = axis.get_legend()
            if legend:
                legend.set_visible(False)
        else:
            leg_spec = self.legend_specs[self.legend_position]
            if self.legend_cols: leg_spec['ncol'] = self.legend_cols
            leg = axis.legend(list(data.keys()), list(data.values()),
                              title=title, scatterpoints=1,
                              **dict(leg_spec, **self._fontsize('legend')))
            title_fontsize = self._fontsize('legend_title')
            if title_fontsize:
                leg.get_title().set_fontsize(title_fontsize['fontsize'])
            frame = leg.get_frame()
            frame.set_facecolor('1.0')
            frame.set_edgecolor('0.0')
            frame.set_linewidth('1.0')
            leg.set_zorder(10e6)
            self.handles['legend'] = leg
            self.handles['bbox_extra_artists'].append(leg)
        self.handles['legend_data'] = data