A "Hookup Language" is a VisualLanguage (but not necessarily a GraphicLanguage, because SignLanguage is a HookupLanguage in many situations) where you can HookUp symbols to one another. The spatial juxtaposition of symbols means something.
The positioning of the symbols means more than just, "the past is on the left, and the future is on the right," like we find in a ConventionalTextLayout (such as EnglishLayout or ChineseLayout.)
One good example of this is the SigmaHookupExample, from mathematics: 
The HookupPoints here are the section in green, where you can attach other symbols.
Another good example is ElephantsMemory, which is a HookupLanguage.
SignLanguage is an example of a HookupLanguage that is not a GraphicLanguage. In SignLanguage, people position what they are talking about on a sort of "stage" in mid-air, and then can position things relative to the items on the stage, to explain relationships.
Chinese is generally not a HookupLanguage, even though tit features HookupsInsideTheSymbols. The spatial relationships of the ChineseRadicals within the ChineseCharacters are very important in determining, "Just what does this individual character mean?" However, there are rarely HookUps once you are beyond the confines of a single character, a single UnitSign.
HookupLanguage is very similar to, but subtley different, than DocumentNavigation. For instance, by convention, when we put a level 2 heading, we mean to say, "The stuff beneath this is all related to what I am saying..." DocumentNavigation signs are something of a HookupLanguage.
UniversalModelingLanguage is a HookupLanguage, where the placement of things means something. Another example are ElectricalSchematics.
"HookupLanguage" is one of our WordsForDescribingSymbolLayout.
Discussion
Phew! Been meaning to write this for a while... -- LionKimbro 2004-06-09 16:50:34