code: function checkGraphvizInstalled(config) {
  	if (config.graphVizPath) {
  		const cmd = path.join(config.graphVizPath, 'gvpr -V');
  		return exec(cmd)
  			.catch(() => {
  				throw new Error('Could not execute ' + cmd);
  			});
  	}
  
  	return exec('gvpr -V')
  		.catch((error) => {
  			throw new Error('Graphviz could not be found. Ensure that "gvpr" is in your $PATH.\n' + error);
  		});
  }
docs: Check if Graphviz is installed on the system.
  @param  {Object} config
  @return {Promise}