rashcourse/tooltips/">tooltips &amp; regions</a> system in <a href="http://mapbox.com/tilemill/">TileMill</a>. Under the hood, it&#39;s powered by the open <a href="https://github.com/mapbox/utfgrid-spec/">UTFGrid specification</a>.</p>
<table>
<thead>
<tr>
<th>Options</th>
<th>Value</th>
<th>Description</th>
</tr>
</thead>
<tbody>
<tr>
<td>layer</td>
<td><code><a href="/mapbox.js/api/v2.1.7/l-mapbox-gridlayer">L.mapbox.gridLayer</a></code></td>
<td>The first argument must be a layer created with <code><a h