The current form of collaboration in Qlik Sense only allows for saving of bookmarks for selections, creating and sharing of custom sheets, stories, etc.. The ability to annotate within Qlik Sense is not currently available natively though may change sometime in the future.
That said, a Technology Partner called Metric Insights have developed a solution called Push Intelligence that allows for user collaboration that allows users to leave notes. You may want to explore their solution and see if it's viable for you.
A final option is would be to explore the Qlik Sense API in the event that you would like to develop the a solution for this use case and/or other use cases. Qlik Sense is built on a platform that allows for web developers to build and implement solutions for the environment without having to wait for Qlik to build it first.
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You could consider using Qlik Sense Stories. You can Snapshot your chart, then add to a Story sheet. Drag and drop a text item and annotate the chart. This also provides exporting to PowerPoint and PDF. Another option is to include a Text item below the chart. Another option is to include an HTML object via the use of DevHub to introduce a text object.