A parent Job is a Job that runs one or several subjobs. These subjobs are called child Jobs. The parent Job (or master Job) contains the tRunJob component.
If the Job you selected on the Job Information table is a parent Job, the Charts view changes to a Bar chart stacking all child Jobs above the parent Job.
This Stacked bar graph shows obvious differences in respective time execution and thus help comparing the master Jobs and its child Jobs.
The vertical (y) axis of the chart shows the scale of the time (in seconds) elapsed for the overall execution of the master Job selected including its related child Jobs.
The horizontal (x) axis of the chart shows the moment (date and time) of each execution allowing you to see the evolution and possible changes over a set amount of days.