[multi-chart] Legend Keys are displaying incorrectly

[rwspielman]

Looks like this line is the cause

superset/superset/viz.py

Line 1459 in 94b6b29

"key": prefix + ", ".join(series["key"]),

Expected results

Legends to show up as normal text

Actual results

chart keys show up as tuples

Screenshots

How to reproduce the bug

1. Create 2 charts
2. make a multi line chart
3. observe legends keys displaying incorrectly

Environment

(please complete the following information):

• superset version: master 9fa52d3
• python version: 3.8
• node.js version: 12

Checklist

Make sure to follow these steps before submitting your issue - thank you!

• I have checked the superset logs for python stacktraces and included it here as text if there are any.
• I have reproduced the issue with at least the latest released version of superset.
• I have checked the issue tracker for the same issue and I haven't found one similar.

Additional context

[rwspielman]

#12660

[junlincc]

@rwspielman thanks for reporting! I was able to reproduce. tagged good first issue
We currently are short for help and if you already have a solution, we absolutely welcome your contribution. 🙏

[Rishabhraghwendra18]

Hey!!! May I work on this issue ?

[junlincc]

@Rishabhraghwendra18 Absolutely!thank you for contribution in advance! if you are interested in working on more Superset dataviz project, please let me know. apache-superset/superset-roadmap#150 🙏

[Rishabhraghwendra18]

@junlincc thanks buddy