The following comic from xfcd has been making its way around the data visualization blogs and I couldn't help but repost it here.
The lesson is a good one: every axis should have a label - no exceptions! (Ok, one exception: if the values are January, February, March, ..., you probably don't need to label the axis "months", but anything less explicit than that simply must be labeled!)
The lack of a label, even if you think it's obvious from context, leaves space for your audience to question what they are looking at. If you state it explicitly with an axis label, rather than spend their brainpower trying to figure out what the axis represents, your audience can spend that power on actually understanding the information that is being presented in the graph. Wouldn't you rather that be the case?