[1.3] Move gantt-chart out from subdirectory (#321)

[joshuali925]

Signed-off-by: Joshua Li [email protected]
(cherry picked from commit 12ad9b7) (cherry picked from commit af334ca)

Description

backport #323

Issues Resolved

[List any issues this PR will resolve]

Check List

• New functionality includes testing.
  • All tests pass, including unit test, integration test and doctest
• New functionality has been documented.
  • New functionality has javadoc added
  • New functionality has user manual doc added
• Commits are signed per the DCO using --signoff

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (99b9779) 95.30% compared to head (6614bff) 95.30%.

Additional details and impacted files

@@           Coverage Diff           @@
##              1.3     #325   +/-   ##
=======================================
  Coverage   95.30%   95.30%           
=======================================
  Files           5        5           
  Lines         149      149           
  Branches       45       45           
=======================================
  Hits          142      142           
  Misses          7        7           

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