fix(pandas-postprocessing): percentage compare to use correct column

[villebro]

SUMMARY

Currently the percentage compare post processing operation is using the following formula

change = (new - old) / new

while the correct formula should most likely be

change = (new - old) / old

Example:

increase from 500 to 1000 (expected result is it should have doubled):

old:

change = (1000 - 500) / 1000 = 50 %

new:

change = (1000 - 500) / 500 = 100 %

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TESTING INSTRUCTIONS

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[zhaoyongjie]

LGTM

[codecov]

Codecov Report

Merging #16716 (a7523ac) into master (092ef5b) will decrease coverage by 0.12%.
The diff coverage is 80.00%.

❗ Current head a7523ac differs from pull request most recent head 5a31187. Consider uploading reports for the commit 5a31187 to get more accurate results

@@            Coverage Diff             @@
##           master   #16716      +/-   ##
==========================================
- Coverage   76.99%   76.86%   -0.13%     
==========================================
  Files        1007     1007              
  Lines       54133    54141       +8     
  Branches     7374     7374              
==========================================
- Hits        41678    41616      -62     
- Misses      12215    12285      +70     
  Partials      240      240              

Flag	Coverage Δ
hive	?
mysql	81.75% <80.00%> (+0.04%)	⬆️
postgres	81.81% <80.00%> (+<0.01%)	⬆️
presto	81.62% <80.00%> (-0.04%)	⬇️
python	82.08% <80.00%> (-0.25%)	⬇️
sqlite	81.38% <80.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
superset/utils/pandas_postprocessing.py	84.09% <ø> (ø)
superset/db_engine_specs/bigquery.py	86.84% <66.66%> (-0.92%)	⬇️
superset/models/core.py	89.82% <100.00%> (+0.07%)	⬆️
superset/db_engines/hive.py	0.00% <0.00%> (-82.15%)	⬇️
superset/db_engine_specs/hive.py	69.80% <0.00%> (-16.87%)	⬇️
superset/connectors/sqla/models.py	89.46% <0.00%> (-0.24%)	⬇️
superset/utils/core.py	89.76% <0.00%> (-0.13%)	⬇️
superset/db_engine_specs/base.py	88.58% <0.00%> (+0.19%)	⬆️

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