Arc is essentially broken

[qnzhou]

I have been exploring altair/vega-lite's capability of creating various radial charts, however, I quickly realized that it is incapable of making even moderately complex radial charts due to multiple bugs:

• Stack setting is not respected: Radial chart radius field unexpectedly changes arc stack behavior #7957
• Arc does not work with facet: facet fails on arc #8754
• Arc does not work with concat either: single arc in concat not rendering #7856

Is there plan of fixing them? Radial chart is an important class in data visualization, it will be pity if radial chart is not supported due to bugs.