code: def _merge_tools(self, subplot):
          
          if self.batched and 'hover' in subplot.handles:
              self.handles['hover'] = subplot.handles['hover']
          elif 'hover' in subplot.handles and 'hover_tools' in self.handles:
              hover = subplot.handles['hover']
              # Datetime formatter may have been applied, remove _dt_strings
              # to match on the hover tooltips, then merge tool renderers
              if hover.tooltips and not isinstance(hover.tooltips, util.basestring):
                  tooltips = tuple((name, spec.replace('_dt_strings', ''))
                                    for name, spec in hover.tooltips)
              else:
                  tooltips = ()
              tool = self.handles['hover_tools'].get(tooltips)
              if tool:
                  tool_renderers = [] if tool.renderers == 'auto' else tool.renderers
                  hover_renderers = [] if hover.renderers == 'auto' else hover.renderers
                  renderers = tool_renderers + hover_renderers
                  tool.renderers = list(util.unique_iterator(renderers))
              if 'hover' not in self.handles:
                  self.handles['hover'] = tool
docs: Merges tools on the overlay with those on the subplots.