Top Sessions Graph - response (1) by gryback

The Top Sessions graph only plots out information on one metric, not two. The bar graph is used to display sessions with the largest values of the selected metric to be blocks with a relative size of the metric value. The color / shading is just used to highlight for differentation of the different blocks going from darket (highest metric value) to lightest (lesser metric value).  
Regarding thresholds, you cannot set these within the sessions graph bar but can in the table display below through table actions - configure columns - set thresholds. This will color highlight the outlier metrics exceeding the set threshold.