polygerrit-ui/app/samples/suggest-vote.html

<dom-module id="suggested-vote">
  <script>
    Gerrit.install(plugin => {
      const replyApi = plugin.changeReply();
      let wasSuggested = false;
      plugin.on('showchange', () => {
        wasSuggested = false;
      });
      const CODE_REVIEW = 'Code-Review';
      replyApi.addLabelValuesChangedCallback(({name, value}) => {
        if (wasSuggested && name === CODE_REVIEW) {
          replyApi.showMessage('');
          wasSuggested = false;
        } else if (replyApi.getLabelValue(CODE_REVIEW) === '+1' &&
            !wasSuggested) {
          replyApi.setLabelValue(CODE_REVIEW, '+2');
          replyApi.showMessage(`Suggested ${CODE_REVIEW} upgrade: +2`);
          wasSuggested = true;
        }
      });
    });
  </script>
</dom-module>