Speaking of 'being lower than the reference level' rather than the graph itself.
I recommend you research what a decibel is, how it works, then revisit this question.
Generally, digital audio samples are measured with reference to "full scale" which allows it to be measured with a unit dBFS which means "decibels with reference to full scale". 0dBFS is the largest sample value possible, therefore peak.
Consequently, every sample value that isn't peak will be metered using a "negative" dBFS value, which indicates it's "below full scale".