docs: function(ctrName, valueName) {
        var ctr = this.model_.kernel
          .getOrCreateCounter(null, 'vreg ' + ctrName + ' ' + valueName);
        // Initialize the counter's series fields if needed.
        if (ctr.series[0] === undefined) {
          ctr.addSeries(new tr.model.CounterSeries(valueName,
          ColorScheme.getColorIdForGeneralPurposeString(
          ctrName + '.' + valueName)));
        }
        return ctr;
      }