docs: def toggle_deriv(self, evt=None, value=None):
          "toggle derivative of data"
          if value is None:
              self.conf.data_deriv = not self.conf.data_deriv
  
              expr = self.conf.data_expr or ''
              if self.conf.data_deriv:
                  expr = "deriv(%s)" % expr
              self.write_message("plotting %s" % expr, panel=0)
  
              self.conf.process_data()