Right next to the About section is an icon with three stacked squares. This is the Category Activation section, which allows you to activate or ‘turn on’ and deactivate or ‘turn off’ the layers that are displayed on the map.
Each category is essentially a layer of elements on the map.
By default, only the category of Absent Elements is activated. Hence, only the elements of the category of Absent Elements are displayed.
Click on the little arrow beside the name of a category to expand the category (or layer) and view the legend of the elements of that category. Click on the arrow again to collapse the category.
If you wish to deactivate a category and activate a different one, uncheck or check the box to the left of name of the category.
You can also expand and collapse all categories, and activate and deactivate categories (or turn on and off all layers) through a single click by clicking on the icon to the extreme right of the Category Activation section.