fix: Tooltip of area chart shows undefined total

[michael-s-molina]

SUMMARY

Fixes a bug where the tooltip of the area chart displays undefined totals.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TESTING INSTRUCTIONS

Make sure the total are correctly calculated.

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

[codecov]

Codecov Report

Merging #24916 (35faf20) into master (85a7d5c) will decrease coverage by 0.01%.
Report is 1 commits behind head on master.
The diff coverage is 0.00%.

❗ Current head 35faf20 differs from pull request most recent head 34308fe. Consider uploading reports for the commit 34308fe to get more accurate results

@@            Coverage Diff             @@
##           master   #24916      +/-   ##
==========================================
- Coverage   69.00%   68.99%   -0.01%     
==========================================
  Files        1906     1906              
  Lines       74134    74135       +1     
  Branches     8208     8208              
==========================================
  Hits        51153    51153              
- Misses      20858    20859       +1     
  Partials     2123     2123              

Flag	Coverage Δ
javascript	55.83% <0.00%> (-0.01%)	⬇️

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

Files Changed	Coverage Δ
...tend/plugins/legacy-preset-chart-nvd3/src/utils.js	15.82% <0.00%> (-0.11%)	⬇️

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[villebro]

LGTM