Just posted about this in another old thread, thought it worth sharing here as well (hope that is within the rules) for all the googlers trying to figure out how to solve this one...
Old topic but in case others are stuck on this I wanted to share another solution (perhaps variation on previous solutions) that worked for my case.
Assuming your chart Items property points to a collection (or involves a collection as a datasource) then adding and then removing a row of 'dummy data' on the collection will reset / deselect the selected segment of the chart.
This is the example, which may need adjustment if your collection doesn't have a column named 'Title' (in which case just replace this section with a text column that does appear in your collection.
This worked for me because I specifically wanted the reset / deselect to be attached to another button in the screen which performed other actions that I won't go into. Either way perhaps the principle of why it worked - that changes to the datasource or refresh of datasource triggers chart to reset 'selected' properties - may assist in hacking your solution to the challenge.
Collect(
YOURCOLLECTIONNAME,
{Title: "REMOVE"}
);
Remove(
YOURCOLLECTIONNAME,
LookUp(
YOURCOLLECTIONNAME,
Title = "REMOVE"
)
)
