Gantt Cloud use standard JIRA Issue's time estimates when rendering the chart. Width of an issue and color depends on provided time estimates and logged work. You can see in example below how time tracking reported for an issue in JIRA (Fig.2) correspond with rendered chart (Fig.1):

Notes

Issues without time estimates will always be rendered with a transparent background and fixed width approximate to half of a day (see: EP-5 issue above as an example). Additional warning icon will also be visible in the left header.
You can change how many working hours should be taken into account when calculating width in add-on's configuration dialog.
It is OK to have Stories and Epics that doesn't have their own time estimates (seeL EP-1 Story above). In such case, parent issue will span over all it's subtasks. Read more about rendering Stories and Epics.

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