Though fierarul's comment leads to a natural question. The original comment explaining the graph is really necessary for the link to make sense. Explaining the graph in a comment seems to break the (apparent) convention of submissions being self-contained.
I thought the reason for disallowing both a URL and text in a submission was to avoid this. Though, if a submission that requires a comment is acceptable, why not allow submitting both a URL and text?
There is a difference. A comment is voted up at the discretion of the users, while text associated with the url would be displayed at the top no matter what. Which would mean the first person who submitted a link could essentially make their comment upon it always be the top one, and define the discussion.
I think you're missing the point. Imagine if everyone on hn posted a link that directed people to half of the content, and put the other half in a comment. That would make for a pretty crappy trend. Note that your original comment has 25 points, so as you can see there's incentive for people to start doing this.
I think a non-vote-able comment at the top wouldn't do as much damage as you might think. Maybe, instead, make it a rule where half & halfs get filtered/blocked.
I thought the reason for disallowing both a URL and text in a submission was to avoid this. Though, if a submission that requires a comment is acceptable, why not allow submitting both a URL and text?