documentclass{standalone}
usepackage{pgfplots}
pgfplotsset{compat=1.15}

pgfplotstableread{
x y z
5 5 1
6 6 2
7 7 3
8 8 4
}mytable

begin{document}

begin{tikzpicture}
begin{axis}[
    colorbar,
    colorbar style={ytick={1,2,3,4},ylabel={z}},
    legend style={at={(current bounding box.north)},anchor={south}}
]

addplot [scatter,scatter src=thisrow{z},only marks,mark=*] table {mytable};

legend{loooooooooooooooooooooooooong entry}

end{axis}
end{tikzpicture}

end{document}