?? yields an error on my iPad...

what would be the proper way to add text label to each of the dots?

Add a text mark: https://observablehq.com/@observablehq/plot-text

got it, thanks!

Don't think this link leads to a viewable notebook

Oops, fixed! https://observablehq.com/@observablehq/qq-plot

Is it possible to have a legend for the data shown on circle radius? Thanks!

it's on the todo-list https://github.com/observablehq/plot/issues/236; in the meantime you have to create the legend yourself (see examples in the related issue)