Sometimes it can happen that the visualization of the Cluster or Materials analysis display an item with very small sub-items where the labels are not visible. To get an overview of what these items exactly are, you can zoom in on that specific item.
To do this, you need to click and hold down your left mouse button (+/- 1 second) on the item to activate the zoom. An expanded item is indicated by several arrows. To zoom out, just click and hold again on that item.