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}
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}[]
\end{axis}
\end{tikzpicture}

\end{document}


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

Would it be an option to use nodes near coords instead?

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

\begin{document}

\begin{tikzpicture}
\begin{axis}