handle selection
    when a selected item is added to a cluster, update the item's delete button and dragareas
    a selected item should not be clustered, unless when zooming in/out again
    -> when an item is changed/removed, set a flag "needRecluster" true,
       and apply reclustering as soon as the user zooms in/out?

make clusters non-editable but selectable
    when a cluster is selected, select all containing items.

items must be reclustered after an item has been moved/changed

optimize
    handle change of data (also individual items) -> use item.changed

create an option to define a cluster method, which has the items to be clustered
    as input, and the data for the clustered item as output
create an option to define cluster levels yourself



double clicking on a cluster should zoom in onto this cluster?
    -> you should be able to easily see the contents of the cluster

autoHeight with ranges: does not take into account a margin around the range


EXTRA

stacking is not "stable", items can continuously switch order when moving or zooming
rename size.frameWidth to size.frame.width, same with height, etc.