docs: def _add_single_metric(self, timestamp, metric_name, value):
          """Store a single metric in the _data dict which can be
          converted to a dataframe.
          """
          # note that this method is built this way to make it possible to
          # support live-refreshing charts in Bokeh at some point in the future.
          self._data['timestamp'].append(timestamp)
          self._data['metric_name'].append(metric_name)
          self._data['value'].append(value)