documentclass[tikz]{standalone}
usepackage{pgfplots}
usepackage{pgfplotstable}
begin{document}

pgfplotstableread{
  1
  2.5
  3
} myTable % -- I can't use this method with the `expr` syntax ?

begin{tikzpicture}
  begin{axis}
    [
      xtick = {1,2.5,3},
      ytick = {1,6.25,9},
      domain = 0:4,
      axis lines = middle,
    ]
    addplot [thick, blue] {x^2} node [near end, anchor = east] {$y = x^2$};
    addplot [xcomb, red] table[x index = 0,y expr=x^2] {
        1
      2.5
      3
    };
    addplot [ycomb, red] table[x index = 0,y expr=x^2] {
        1
      2.5
      3
    };
    %addplot table[x index=0,y expr=x^2] {myTable}; % does not work ?!
  end{axis}
end{tikzpicture}
end{document}

draw [myStyle] (axis cs: 1, 0) |- (axis cs: 0, 1) ;
draw [myStyle] (axis cs: 2.5, 0) |- (axis cs: 0, 6.25) ;