You can create a group of issues on the chart that will be "wrapped" by parent task (summary element) and rendered underneath of it (Fig.1). There are two possible ways to achieve such effect. First one, is creating a Story issue type and then adding sub-tasks to it, second method is using Epic. Each issue added to the Epic will be treated as a subtask and rendered in exactly the same manner as stories and sub-tasks.
In above example Issue EP-10, which is a Story, doesn't have it's own estimates so it's duration is calculated based on two sub-task issues: EP-11 and EP-12. Note that issue EP-12 is also connected with EP-11 using "blocked by" link. Read more about creating dependencies between issues.