% used PGFPlots v1.14
documentclass[border=5pt]{standalone}
usepackage{pgfplots}
    usetikzlibrary{
        fit,
        shapes,
        pgfplots.groupplots,
    }
        newcommand{dsnfive}{prob5_2.dat}
    begin{filecontents*}{dsnfive}
        2   7   4   8
        3   6   5   8
        2   5   9   5
        3   5   9   9
        3   3   9   4
        2   2   8   9
        5   1   8   8
        6   2   6   9
        8   1   7   4
        6   4   4   4
    end{filecontents*}
begin{document}
    begin{tikzpicture}
        begin{axis}[
            xmin=1,
            xmax=10,
            ymin=0,
            ymax=10,
            legend pos=outer north east,    % <-- used predefined command to position legend
            only marks,
            mark=*,
            mark options={
                scale=1.2,
            },
        ]
                % store number of data points
                pgfplotstablegetrowsof{dsnfive}
                pgfmathtruncatemacro{N}{pgfplotsretval-1}

            addplot+ [
                blue,
                % -------------------------------------------------------------
                % comment this block to hide names of the coords
                % -----
                % text of node coords should be the same as for the
                % corresponding coordinates
                nodes near coords=acoordindex,
                % increase the distance of the nodes a bit
                nodes near coords align={above=1ex},
                % -------------------------------------------------------------
            ] table [x index=0,y index=1] {dsnfive}
                % set a coordinate on each data point
                % (needed for the `fit' library solution)
                foreach i in {0,...,N} {
                    coordinate [pos=i/N] (ai)
                }
            ;
                addlegendentry{$class 1$}
            addplot+ [
                red,
                % -------------------------------------------------------------
                nodes near coords=bcoordindex,
                nodes near coords align={below=1ex},
                % -------------------------------------------------------------
            ] table [x index=2,y index=3] {dsnfive}
                % set a coordinate on each data point
                % (needed for the `fit' library solution)
                foreach i in {0,...,N} {
                    coordinate [pos=i/N] (bi)
                }
            ;
                addlegendentry{$class 2$}
                addplot[]coordinates{(2,7)}node[pos=0](A){};
               node[very thick, draw=green!75!black,circle,fit=(A), fill = red, opacity = 0.2]{};
end{axis}
            % now add the circles to the points
            foreach i in {a0,b6,b8} {
                draw [green!60!black,thick] (i) circle (5pt);
            }
    end{tikzpicture}
end{document}