I recently found out about the graphic notation. I was reading up on it on wikipedia, but there was very little information about how the scores are read besides you read them left to right. There is an example score on the wikipedia page that looks extremely interesting. How would someone interpret this notation?
Graphic notation is a fascinating topic. There's no one type of graphic notation, so there's no one answer about how to read it. For example, George Crumb sometimes uses traditional notation, but bent into different shapes—sometimes in order to define complex formal schemes, sometimes just to make form more visually immediate, and sometimes primarily for extra-musical reasons.
An example of the first would be a score in which one player plays a continuous musical loop represented as a circle they could read (theoretically) for eternity while other players are playing more linear music that doesn't line up with it in any particular way. This would be very difficult to notate in a purely traditional way since the elements won't line up the same way on any two performances. I'm not 100% certain, but I believe the piece Starchild has some sections that work like this. [EDIT: I finally got ahold of the Star-Child score today, and I can now confirm that this is indeed how the circles work in that piece. It's a beautiful score in which several independent cycles move around each other until the ending. ]
An example of the second would be his score for the String Quartet Black Angels, which includes a number of places in which the players move in and out of unison with each other. He notates these moments as staves colliding or splitting in the score. This mostly could have been written in a traditional way, but the form and the nature of the instrumental interactions is more viscerally obvious.
An example of the third type would be the Spiral Galaxy movement of his Makrokosmos for piano. The score is bent into a spiral shape, but that's mostly just for the sheer beauty of the music on the page.
The Steiner example you linked to is to some extent just a graphic representation of an electronic piece, so the performer reads it while performing a Pd patch. According to Steiner's website, time moves from left-to-right. There are two rows, the top row is bigger and uses brighter colors, while the bottom row is shorter and uses darker shades of the same colors. The specific colors represent different samples, something that would presumably be obvious in the Pd patch itself. The smaller, bottom row has drawings of the general volume and pan settings, while the specific shape in the upper row visually represents what parts of the sample are being played at any given moment. Apparently that last part is represented from bottom of the shape to the top, but I don't quite understand how that works. This score works more as a mnemonic device for a laptop performance using a very particular setup that isn't specified in the score, so its performative use is probably limited to the composer himself. For a listener however, following along in the score does give you a sense of how the pieces are fitting together, at least in a general sense. It helps to delineate between the different samples, and the bottom row gives a nice visual of the ways that the sounds move around in the stereophonic space.
Some composers (Anthony Braxton for example) invent their own vocabulary of shapes and then teach the performers what kinds of sounds the different shapes represent. Some composers design graphically evocative scores, but want the performers to improvise based on whatever the shapes and/or colors inspire in them individually. Some composers (including Morton Feldman, John Cage and Christian Wolff) devise graphic scores in order to either (or both) be less specific about musical elements that traditional notation is usually very specific about (such as pitch and rhythm), and to be more specific about musical elements that are poorly represented in traditional notation (such as timbre and complex or even random metric alignments). I put some of these examples and a few others in an article I wrote for the New York Times website here, which might get you started on some of the possibilities out there.