And still on the topic of LaTeX presentations, this time trying to plot a symbol over a bar to indicate significance.

This is how it works:

\node[xshift=\pgfkeysvalueof{/pgf/bar shift},anchor=south] at (axis cs:Xcoord1,0.47) {$\bullet$}; 

You need to put this code directly after the point where the data series has been plotted. Example:

\begin{tikzpicture}
\begin{axis}[xtick=data,axis x line*=bottom,axis y line=left,symbolic x coords={Xcoord1, Xcoord2}]

\addplot [ybar,seagreen] coordinates {(Xcoord1, -0.027) (Xcoord2, 0.436)}; 
\node[xshift=\pgfkeysvalueof{/pgf/bar shift},anchor=south] at (axis cs:Xcoord2,0.47) {$\bullet$}; 
\addlegendentry{System 1}

\addplot+ [ybar,blue] coordinates  {(Xcoord1, 0.331) (Xcoord2, 0.095)}; 
\node[xshift=\pgfkeysvalueof{/pgf/bar shift},anchor=south] at (axis cs:Xcoord1,0.36) {$\bullet$};
\addlegendentry{System 2}

\addplot+ [ybar,orange] coordinates {(Xcoord1, 0.222) (Xcoord2, 0.441)}; 
\node[xshift=\pgfkeysvalueof{/pgf/bar shift},anchor=south] at (axis cs:Xcoord1,0.25) {$\bullet$};
\node[xshift=\pgfkeysvalueof{/pgf/bar shift},anchor=south] at (axis cs:Xcoord2,0.47) {$\bullet$};
\addlegendentry{System 3}
\end{axis}
\end{tikzpicture}