Fix plot tasks
Created by: derkling
The current version of plot tasks is broken in case a task name matches multiple PIDs. In that case a plot:
- report in the title just the first PID
- mix data for all PIDs in the same plot
This patch fixes this issue by actually producing a set of plots for each different PIDs which match a specified task name. Moreover, it is now possible to define the list of tasks to plot by specifying either task names and/or PIDs.