fix(plugin-chart-echarts): use verbose x-axis name when defined

[villebro]

SUMMARY

AFTER

Now x-axis columns with a verbose name work as expected:

BEFORE

Before the x-axis in the series data would be undefined, causing a broken chart:

TESTING INSTRUCTIONS

1. Enable GENERIC_AXES feature flag
2. Open the FCC examples dataset
3. Create an ECharts Line, Bar or other similar chart with "Ethnic minority" sa the x-axis.

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 #18217 (c22059a) into master (b2bfb92) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master   #18217   +/-   ##
=======================================
  Coverage   66.05%   66.05%           
=======================================
  Files        1591     1591           
  Lines       62423    62423           
  Branches     6287     6287           
=======================================
  Hits        41233    41233           
  Misses      19568    19568           
  Partials     1622     1622           

Flag	Coverage Δ
javascript	50.86% <100.00%> (ø)

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

Impacted Files	Coverage Δ
...gin-chart-echarts/src/Timeseries/transformProps.ts	58.06% <100.00%> (ø)

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