I'm not quite sure what you mean by "no page is likely to last more than five years," because a large portion of the Web is composed of pages that, once posted, remain indefinitely. If your site is one of them, you wouldn't want to depend on an external source for your charts, or your archived pages could be broken by a third party--which seems to be your (very good) point, so I'm a bit confused by your wording.
I'm assuming that a page has a life, I picked five years. The implication being if your page isn't going to last long, it's not that big a deal if you rely on an external resource. But if you're one of the few (or the many) that are going to last longer than the expected life of one of these ephemeral services, you shouldn't rely on it. That's as opposed to paying for the services of, say, an established, paid and hopefully long lived CDN or similar.
