Bipolar disorder is not just considered a spectrum disorder by patients, the idea of it being considered on a spectrum is commonly discussed in the professional psychiatric community. You can find information on the bipolar spectrum in books on bipolar disorder and scholarly journal articles.

In a couple articles written by MDs, I have read that at the lowest end of a bipolar spectrum a person may hardly even have had a hypomania. But what the doctor felt made the case qualify was the patient's tendency to have reoccurring depressions followed by more stable periods. The cyclical pattern was the key. Good response to moodstabilizers by such patients was also mentioned. But of course there are different views on what characteristics must be had to qualify for at least the lowest end.