Hey there thank you for posting in the Lucid community! Our sincere apologies for the delayed response on this thread--I wanted to follow up in case this answer may still be useful to you and to provide a solution for other users who see this thread in the future. 

There isn't a way to flip a mirror image of a line but you can reverse the direction of the line is using the "Swap Line Ending" tool (details here). Hope this helps!