I've used this Leaflet add-on but not in Grav and I have never used Grav's leaflet plugin either.
Sorry if I am telling you what you already know. To use Leaflet.markercluster, you need to include the JS library. Let's just do that the simplest way for this example, by including the script from CDN (https://unpkg.com/[email protected]/dist/ as per its README). So you want a <script src=" .." .. ></script> in the HTML head.
OK having started to talk you through this, I can see that this plugin is not like most plugins and seems like it would be very difficult to extend! It also looks like you can only add one marker per map.
Could you explain how you are using this plugin to get more than one marker on a map? That might help me see something I'm missing and I may be able to help you from there.
You can probably override the Twig template in the plugin, but I don't see how you can:
- get more than one marker into the Twig function
- avoid harcdoding your cluster information into the template (it would be much better to be able to pass these variables when calling the Twig function)