The only problem with calculating the ratio by using only the first and latest releases is that it puts too much weight on the last release. If the last release was a slow one (in terms of storyline progression) then the line will be estimated too shallow. The graph interpolates ALL points so that any inconsistencies are averaged out.

As for the chapters instead of volumes, i guess i could do that...its just a straight mapping right? Is that more understandable to manga readers? (I wouldn't know, I'm a manga virgin).

It doesn't update every week. The way I create it is by using release dates on narutofan.com, but the dates are only for the volumes. So, I can only add a datapoint to the anime line when it reaches the end of a certain volume. I could use a smaller granularity (i.e. chapters) but that would take waay too long. I think the anime has reached another volume so I could add one more data point in there. ANYways, the point is that at most I could only update it like every 1-2 months, when the anime reaches the end of a new volume.