Make @df support string column names

[ron-wolf]

Description

Add a special case to add_sym! to convert an AbstractString column name to a Symbol before adding it.

The old behavior would iterate over the string and try to add each character as a column name. This produced a stack overflow, since add_sym! had no special case for Char, resulting in indefinite recursion. More details in JuliaPlots/StatsPlots.jl#562.

Attribution

• I am listed in .zenodo.json (see How to get acknowledged #3503)

Things to consider

• Does it work on log scales? -- Change only affects column names
• Does it work in layouts? -- ''
• Does it work in recipes? -- ''
• Does it work with multiple series in one call? -- No new functions introduced
• PR includes or updates tests? -- No changes to tests
• PR includes or updates documentation? -- No changes to docs

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 78.40%. Comparing base (f017272) to head (56514c7).
Report is 1 commits behind head on v2.

Files with missing lines	Patch %	Lines
StatsPlots/src/df.jl	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##               v2    #5033      +/-   ##
==========================================
- Coverage   78.41%   78.40%   -0.01%     
==========================================
  Files          78       78              
  Lines        8261     8262       +1     
==========================================
  Hits         6478     6478              
- Misses       1783     1784       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

---

🚨 Try these New Features:

• Flaky Tests Detection - Detect and resolve failed and flaky tests

[BeastyBlacksmith]

Great, thanks!