Ordinate X (Date) - relative time of recent update of Web Site / relative time of recent update of a group of Web Sites.
Ordinate Y (Size) - may be the quantity of letters, pages, outside links, pictures, binaries, error links, or given keywords in the HTML pages of Web Site / a group of Web Sites.
Ordinate Z - item of list.
Brightness (in one-color worlds) - the quantity of links to the outside in the HTML pages of Web Site.