Inspired by a question on the CakePHP Community page on Facebook (https://www.facebook.com/groups/cake.community/permalink/673205099378177/), I created a really quick and dirty solution using ReflectionClass, the dot language, which is used by GraphViz and the online tool http://www.webgraphviz.com/

Controller>ModelRelationVizController.php

<?php
App::uses('AppController', 'Controller');

class ModelRelationVizController extends AppController {

	public $uses = array();

	public function beforeFilter() {
		if (Configure::read('debug') < 1) {
			throw new MethodNotAllowedException(__('Debug setting does not allow access to this url.'));
		}
		parent::beforeFilter();
	}

	public function index() {
		$modelPath = ROOT . DS . APP_DIR . DS . 'Model';
		$skipModels = array('AppModel');
		$modelRelationArray = array();
		$skippedClassCount = 0;
		$hasOneCount = 0;
		$hasManyCount = 0;
		$belongsToCount = 0;
		$hasAndBelongsToManyCount = 0;
		$relationCount = 0;
		$skippedRelationCount = 0;
		$relationTypes = array(
			'hasOne',
			'hasMany',
			'belongsTo',
			'hasAndBelongsToMany',
		);

		$modelFiles = glob($modelPath . '/*.php', GLOB_NOSORT);
		$classCount = count($modelFiles);
		foreach ($modelFiles as $modelFile) {
			$modelClass = basename($modelFile, '.php');
			if (in_array($modelClass, $skipModels)) {
				$skippedClassCount++;
				continue;
			}
			require_once $modelFile;
			$reflectionClass = new ReflectionClass($modelClass);

			$modelRelationArray[$modelClass] = array();
			foreach ($relationTypes as $relationType) {
				if ($reflectionClass->hasProperty($relationType)) {
					$modelRelations = $reflectionClass->getProperty($relationType)->getValue(new $modelClass);
					foreach ($modelRelations as $relationAlias => $modelRelation) {
						$relationCount++;
						if (in_array($modelRelation['className'], $skipModels)) {
							$skippedRelationCount++;
							continue;
						}
						${$relationType . 'Count'} += 1;
						$modelRelationArray[$modelClass][$relationType][$relationAlias] = $modelRelation;
					}
				}
			}
		}

		$this->set('classCount', $classCount);
		$this->set('skippedClassCount', $skippedClassCount);
		$this->set('hasOneCount', $hasOneCount);
		$this->set('hasManyCount', $hasManyCount);
		$this->set('belongsToCount', $belongsToCount);
		$this->set('hasAndBelongsToManyCount', $hasAndBelongsToManyCount);
		$this->set('relationCount', $relationCount);
		$this->set('skippedRelationCount', $skippedRelationCount);
		$this->set('modelRelationArray', $modelRelationArray);
	}
}

View>ModelRelationViz>index.ctp

<?php ?>
<h1>ModelRelationViz</h1>
<h2>Statistics</h2>
<table>
	<tbody>
		<tr>
			<th style='text-align: left;'><?php echo __('Total classes found:'); ?></th>
			<td><?php echo h($classCount); ?></td>
		</tr>
		<tr>
			<th style='text-align: left;'><?php echo __('Classes skipped'); ?></th>
			<td><?php echo h($skippedClassCount); ?></td>
		</tr>
		<tr>
			<th style='text-align: left;'><?php echo __('hasOne relations found:'); ?></th>
			<td><?php echo h($hasOneCount); ?></td>
		</tr>
		<tr>
			<th style='text-align: left;'><?php echo __('hasMany relations found:'); ?></th>
			<td><?php echo h($hasManyCount); ?></td>
		</tr>
		<tr>
			<th style='text-align: left;'><?php echo __('belongsTo relations found:'); ?></th>
			<td><?php echo h($belongsToCount); ?></td>
		</tr>
		<tr>
			<th style='text-align: left;'><?php echo __('hasAndBelongsToMany relations found:'); ?></th>
			<td><?php echo h($hasAndBelongsToManyCount); ?></td>
		</tr>
		<tr>
			<th style='text-align: left;'><?php echo __('Total relations found:'); ?></th>
			<td><?php echo h($relationCount); ?></td>
		</tr>
		<tr>
			<th style='text-align: left;'><?php echo __('Relations skipped'); ?></th>
			<td><?php echo h($skippedRelationCount); ?></td>
		</tr>
	</tbody>
</table>
<h2>GraphViz Data</h2>
<pre>
digraph prof {
ratio = fill;
node [style=filled];

<?php foreach ($modelRelationArray as $modelClassName => $modelRelationTypes):
	$rColor = mt_rand(100, 900) / 1000;
	$gColor = mt_rand(100, 900) / 1000;
	$bColor = mt_rand(100, 900) / 1000;
	echo sprintf('%s [color="%s %s %s"]' . PHP_EOL, $modelClassName, $rColor, $gColor, $bColor);
	foreach($modelRelationTypes as $modelRelationTypeName => $modelRelations):
		foreach ($modelRelations as $relationAlias => $modelRelation):
			echo sprintf('%s -> %s [color="%s %s %s", label = "%s as %s"]' . PHP_EOL,
				$modelClassName,
				$modelRelation['className'],
				$rColor,
				$gColor,
				$bColor,
				$modelRelationTypeName,
				$relationAlias
			);
		endforeach;
	endforeach;
endforeach; ?>
}
</pre>