fix(sankey): Return the exact faulty link instead of root

[nottatdat]

SUMMARY

The current implementation returns the root of the faulty branch in the tree

if neighbour in path or visit(neighbour):
    return (vertex, neighbour)

The goal of this PR is to pinpoint the exact faulty link in the tree.

cycle = (vertex, neighbour) if neighbour in path else visit(neighbour)
if cycle:
    return cycle

[rusackas]

@nottatdat can you give us a way to test this, or screenshots of whatever's broken? I'm assuming this is still an issue in Superset 3.x? Meanwhile, closing/reopening to see if we can get it to pass CI.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (7ef06b0) 65.76% compared to head (cab4668) 68.02%. Report is 1546 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #23444      +/-   ##
==========================================
+ Coverage   65.76%   68.02%   +2.26%     
==========================================
  Files        1908     1909       +1     
  Lines       73726    73906     +180     
  Branches     7989     7989              
==========================================
+ Hits        48489    50278    +1789     
+ Misses      23189    21580    -1609     
  Partials     2048     2048              

Flag	Coverage Δ
hive	53.81% <0.00%> (?)
mysql	78.04% <100.00%> (?)
postgres	78.14% <100.00%> (-0.37%)	:arrow_down:
presto	53.76% <0.00%> (?)
python	83.10% <100.00%> (+4.59%)	:arrow_up:
sqlite	77.66% <100.00%> (?)
unit	56.49% <0.00%> (?)

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