List of Categories used in a Collection
<!--
This loop loops through a collection called `collection_name`
and sorts it by the front matter variable `date` and than filters
the collection with `reverse` in reverse order
To make it work you first have to assign the data to a new string
called `sorted`.
-->
<ul>
{% assign sorted = (site.collection_name | sort: 'date') | reverse %}
{% for item in sorted %}
<li>{{ item.categories }}</li>
{% endfor %}
</ul>