You can tag items in the scatter plot, radar chart or bar plot. It's easiest to do in the scatter plot, where it's possible to select multiple items at once, whereas the others only allow selecting items one at a time.

Open the tag editing window from the Item tags section by pressing the pen icon:

Add a new tag by typing a name for it, selecting a symbol (the symbol is optional) and pressing Add:

When you have at least one tag, you can tag items with it.

Select the items you wish to tag and check the box for each tag you wish to tag them with (an item can be tagged using multiple tags):

NOTE. When you select multiple items, the box for a tag will be checked only when every selected item has been tagged with the that tag. If some have not been tagged, the box will be unchecked. Checking the box will tag the items that weren't yet tagged. Unchecking untags all selected items.