Let custom mark depend on a variable

I am trying to create a custom mark to use in a scatterplot. pgfplots: problem in the legend using custom marks has provided a hint on how to proceed but I do not know how to make this mark depend on a third variable. For example, the code below, is it possible to have pgfdeclareplotmark get the node label (which I have hard-coded with {a} below) from the label column in the data?

documentclass{article}
usepackage{pgfplots}pgfplotsset{compat=1.16}
usepackage{pgfplotstable}
pgfplotstableread{
 y x label
 1 1     a
 2 2     b
 3 3     c
 }testdata
 
pgfdeclareplotmark{myshape} {node[draw, rounded corners=2pt, color=blue, fill=white] {a};}

begin{document}

begin{tikzpicture}
begin{axis}[]
  addplot[ mark=myshape] table[x=x, y=y] {testdata};
end{axis}
end{tikzpicture}

end{document}

This is what I’d like to achieve with the labels. Thank you for any help.

documentclass{article}
usepackage{pgfplotstable}
pgfplotsset{compat=1.16}
pgfplotstableread{
 y x label
 1 1     a
 2 2     b
 3 3     c
 }testdata
 

begin{document}

begin{tikzpicture}
begin{axis}
  addplot[
     point meta=explicit symbolic,
     nodes near coords,
     nodes near coords align=center,
     nodes near coords style={draw, rounded corners=2pt, color=blue, fill=white}
     ] table[x=x, y=y, meta=label] {testdata};
end{axis}
end{tikzpicture}

end{document}