Plots and categorizes blood pressure according to the The Fourth Report on the Diagnosis, Evaluation, and Treatment of High Blood Pressure in Children and Adolescents from National High Blood Pressure Education Program Working Group on High Blood Pressure in Children and Adolescents. "Prehypertension" is from the 90th to 95th percentile, while "hypertension" is anything above the 95th percentile. The report further divides hypertension into "Stage 1", less than the 99th percentile plus 5 mmHg, and "Stage 2" which is anything above that. Stage 2 requires more urgent evaluation and treatment. The data are from tables 3 (boys) and 4 (girls) from that paper.
For scale, the 50th percentile line is shown, with pressures above that labelled "High Normal" and "Low Normal", though there is no clinical importance to that distinction. For systolic blood pressures, the approximate 5th percentile line is also shown. This is the cutoff for decompensated shock in the American Heart Association's Pediatric Advanced Life Support algorithm.
The blood pressure norms are a function of height in the original report, but as Dr. David Kaelber points out, using the 5th percentile height values is simpler and likely as useful. Thus, if the patient's height is not given, the graph uses those norms.