This diagram, by a self appointed Unix Guru in France, illustrates the output of programming language development by plotting the genealogy of the many common and influential programming languages. It tells you the year when a language was launched, what revisions the language experienced, the languages from which it inherited features, and the subsequent languages inheriting its features.
I think the diagram is easy to read. More specifically, while there are many lines in the picture, they are mainly horizontal. Any that aren't tend to the right and generally end with a label. Also, the diagram doesn't require much "arbitrary" knowledge to understand. One must merely understand that a programming language can inherit the features of others and undergo revisions. Finally, while the features of the graph are simple, it remains information rich, telling many small stories and several large ones. The diagram illustrates which languages were very influential (ie Algol) on others and which were not (Prolog). It tells a history of programming language development, how some languages went through many revisions slowly (Fortran) or rapidly (PHP), as well as the more fruitful years of the field.
This picture illustrates the chemistry and forces that produce deep sea vent, also known as a black smoker. It was created by an employee at the US National Oceanic and Atmospheric Administration and was linked to a somewhat educational article on Hydrothermal Vents and ocean chemistry.
I find this diagram to be extremely "busy". There are many classes of lines and arrows, each of them illustrating something different going on. Some of them, particularly the dotted lines of the "spreading axis" remain a mystery to me. Many of the other signifiers on the diagram are inconsistent; some color designates temperature while other color designates material property. Also, there are many different kinds of labels, some for temperature, legend labels, material labels, etc. Finally the diagram requires an exceptional amount of domain specific knowledge and seems to need a 10 to 20 minute narration as there are many elements that no amount of staring will explain. Unfortunately the accompanying article does not provide much detail to this brain-buster of a diagram.