Since it is common for people to be confused with pixel coordinates and Allegro 5's primitive drawing (eg this latest thread), I have created a new article on the Wiki, Drawing Primitives in Grid Coordinates, to add to the mythos.

I wouldn't classify anybody who's run into this specific problem as a "line drawing n00b". Those threads usually have titles like "line drawing bug?" in-so-much that the poster has plenty of experience and confidence in his ability to draw a line.

The problem occurs in the distinction between plotting to pixels and plotting on grids. People who are still struggling need to get to know how coordinates aren't pixels. That's what the article's about. Perhaps I could find a more fitting title that doesn't imply "Now Let's Draw Primitives! Yay!"

I kept the html and color names in because I find them incredibly useful, e.g. I want other people to know they're available. Plus, the article is about how shapes plot differently in grid and pixel coordinates. If they get stuck on how to map a color, then they are a noob and in the wrong tutorial.

That being said, I do need to create a little adjunct article about turning on and off sub-pixel rendering.

First of all: good effort, with nice pictures that should hopefully clarify things for a lot of people.

Now for two points of criticism (yes, I know it's a wiki and I should go and edit it myself, but it's easier to say how to improve something than it is to do it, especially since it'd take a fair bit of my time). The first is that the article is titled "Drawing Primitives in Grid Coordinates", which in itself is fine, but it then goes off and discusses the situation in Allegro 4 in preparation of discussing the situation in Allegro 5. That in itself is also fine if what I'm looking for is a guide on how Allegro 5 is different from Allegro 4, but the title of the article doesn't make it clear that that's what it's going to discuss. I think the article should stand on its own and explain how primitive drawing works in Allegro 5, without referencing how it's done in Allegro 4. Afterall, if I'm looking to learn how it's done in A5 but I don't know A4, I don't want to learn A4 first just so I can learn A5 afterwards.
It might make sense though to have two sections in the article: one discussing Allegro 4, one discussing Allegro 5. Then the current content can become sub-sections under the Allegro 5 heading.
The second point of criticism is that the article assumes we understand what "switch on sub-pixel rendering" means. That could do with a brief introduction of a sentence or two, below this figure
{"name":"A5line2.png","src":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/f\/7\/f76ae2b7d7e0887de93741b27abab1fe.png","w":475,"h":247,"tn":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/f\/7\/f76ae2b7d7e0887de93741b27abab1fe"}

I'm with Matthew on this one: make the examples as to-the-point and clear as you can. That means getting rid of the HTML and colour names. In fact, I'd just use an (implicit) ALLEGRO_COLOR variable called blue or red or yellow in preference of a function call.
Keep the code clean, focussed and as simple as possible.

Could you explain what sub-pixel rendering is (on the wiki)? Its unclear to me if thats an actual mode you can set or just something to make understand how rendering works easier.

Allegro's primitives are not intuitive to someone who only thinks in terms of a 2D library. The article will (may) get them to realize there's something else going on under the hood, but they'll still come out with a very similar question:

"So how do I draw the horizontal line from (0,0) to (x,0)?

"I tried adding 0.5 like the article said al_draw_line(0.5,0.5, x + 0.5, 0.5, red, 1.0), but it still doesn't always work."

I think you give the average Allegro n00b too much credit if you think they will read your article and actually apply the theory into a working application. But again, if the article isn't written with them in mind, it doesn't really matter.

I think the new version is better.
So, some more comments then: the terms "pixel coordinates" and "grid coordinates" are not clearly defined. I can understand what they mean without looking at the pictures, but I'm not the target audience for this article. It's easy to work out by scrolling down to the images and comparing the one that says "in pixel coordinates" and the one that says "in grid coordinates", but without doing that the introduction is still confusing.
I would rewrite the first section to explain and introduce those concepts, then explain how you'd draw a line in pixel coordinates as an illustration. I would not introduce a fictional function that would draw such a line because it doesn't actually make things clearer: you're still introducing a code example that doesn't do what you're trying to explain so that you can later explain the code that you want to explain by pointing out the difference.
I would also leave out the ALLEGRO_COLOR definitions, in a sense they're just bloat that detracts from the main point. In fact, looking at it now I would probably just replace the argument in all of them with a generic variable colour. On its first introduction you can explain that it's an ALLEGRO_COLOR variable and leave it at that. The colours really aren't an essential part of what you're trying to explain here and it's better to be to the point.

I can hear the questions now... how do I set the colour? No, I think it's best to leave in an ALLEGRO_COLOR variable. It only takes a brief explanation to make it clear that is how you set colours. They can learn about other functions to do with colour on their own later. But you want to give new programmers some place to start rather than just leaving a vague reference to colour. It is relevant to the topic as you're not plotting a line without it.