Hi,
The Average Transaction Response Time graph displays the average time taken to perform transactions during each second of the load test scenario run.
And the 90th percentile is the value for which 90% of the data points are smaller.
You can refer some threads as below for more information:
http://support.openview.hp.com/selfsolve/document/KM169562
Transactions/m-p/5643545/highlight/true#M2363
Regards,
Chau