code: def zoom_pinch_cb(self, fitsimage, event):
          
          chviewer = self.fv.getfocus_viewer()
          bd = chviewer.get_bindings()
  
          if hasattr(bd, 'pi_zoom'):
              return bd.pi_zoom(chviewer, event)
  
          return False
docs: Pinch event in the pan window.  Just zoom the channel viewer.