\documentclass[a4paper]{amsart}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage[finnish]{babel}
\usepackage{url}
\usepackage{parskip} % Tyhjä rivi kappaleiden väliin.
\usepackage{booktabs}
\usepackage{tikz,pgfplots}
\renewcommand*\arraystretch{2}
\listfiles
\begin{document}

\begin{tikzpicture}
\begin{axis}[%
scatter/classes={%
    a={mark=o,draw=black}}]
\addplot[scatter,only marks,%
    scatter src=explicit symbolic]%
table[meta=label] {
x y label
1 4.3 a
2 5.1 a
3 5.7 a
4 6.3 a
5 6.8 a
6 7.1 a
7 7.2 a
8 7.2 a
9 7.2 a
10 7.2 a
11 7.5 a
12 7.8 a
    };
\end{axis}
\end{tikzpicture}
\end{document}

