Marker categories is the basic trick of filtering markers. Groups of markers can be switched on and off and the groups have different icons. The classic switching by sidebar checkboxes was chosen.
Shall we load each category from a file of its own or shall we have a single file with category data. We shall provide both ways. The concepts can even be mixed.
First we create a SideBar() object, then we create the needed BarCategory() objects. A category will be created if it is specified in file loading options. Also one is created if a new category name is found from files category cell.
The category checkbox switches display of category entries and its markers. Also visibility of marker image on sidebar is switched making sidebar cleaner.
See the page, play with it and see its source.