% Source: http://tex.stackexchange.com/a/8584/5645
\documentclass[varwidth=true, border=2pt]{standalone}

\usepackage{pgfplots}

\begin{document}
    \begin{tikzpicture}
        \begin{axis}[
            symbolic x coords={a small bar, a medium bar, a large bar},
            xtick=data
          ]
            \addplot[ybar,fill=blue] coordinates {
                (a small bar,   42)
                (a medium bar,  50)
                (a large bar,   80)
            };
        \end{axis}
    \end{tikzpicture}
\end{document}