docs: function checkGauge(method, gauge) {
          if (!gauge) {
              throw new Error('SVOGauge.init: method ' + method +
                              'did not create element gauge.');
          }
          if ('function' !== typeof gauge.getValues) {
              throw new Error('SVOGauge.init: method ' + method +
                              ': gauge missing function getValues.');
          }
          if ('function' !== typeof gauge.enable) {
              throw new Error('SVOGauge.init: method ' + method +
                              ': gauge missing function enable.');
          }
          if ('function' !== typeof gauge.disable) {
              throw new Error('SVOGauge.init: method ' + method +
                              ': gauge missing function disable.');
          }
          if ('function' !== typeof gauge.append) {
              throw new Error('SVOGauge.init: method ' + method +
                              ': gauge missing function append.');
          }
      }