pgfplots complains path is empty

Question

I'm trying to fill an area under a curve drew with pgfplots. I've tried the solution outlined in this answer:

\documentclass{article}
\usepackage{amsmath}
\usepackage[usenames,dvipsnames]{xcolor}
\usepackage{pgfplots}

\colorlet{shadecolor}{gray!10}
\numberwithin{equation}{section}

\pgfplotsset{%
    compat=1.11,
    standard/.style={%
        clip=false,
        mark=none,
        smooth,
        axis x line*=bottom,
        axis y line=left,
        axis x line=middle,
        axis y line=middle,
        enlarge x limits=0.15,
        enlarge y limits=0.15,
        every axis x label/.style={at={(current axis.right of origin)},anchor=north west},
        every axis y label/.style={at={(current axis.above origin)},anchor=north east}
    }
}
\usepgfplotslibrary{fillbetween}

\begin{document}
\begin{figure}[!h]
    \centering
    \begin{tikzpicture}
        \newcommand\gaussian{1.0298076 * exp(-(x-2)^2 / 0.48)}
        \pgfplotsset{ticks=none};

        \begin{axis}[standard,every axis plot post/.append style={%
                domain=0:4,samples=200},
        xlabel={$x$},
        ylabel={$p(x)$},
        enlargelimits=upper]
            \addplot[name path=f,CornflowerBlue]{\gaussian};
            \path[name path=axis] (axis cs:0,0) -- (axis cs:1,0); %%%% The problem
            \addplot[
                thick,
                color=blue,
                fill=blue,
                fill opacity=0.05
            ]
            fill between[
                of=f and axis,
                soft clip={domain=1.6:2.4}
            ];
            \addplot[dashed] coordinates {(2.4,0) (2.4,0.737889)};
            \addplot[dashed] coordinates {(1.6,0) (1.6,0.737889)};
            \node at (axis cs:2.4,0.001) [below] {$\mu + \sigma$};
            \node at (axis cs:1.6,0.001) [below] {$\mu - \sigma$};
        \end{axis}
    \end{tikzpicture}
    \caption{Intervallo di confidenza}\label{fig:distib_norm}
\end{figure}
\end{document}

However, in the logs I see

Package pgf Warning: fill between skipped: the second input path is empty. on input line 55.

The offending line is therefore this one:

\path[name path=axis] (axis cs:0,0) -- (axis cs:1,0);

How can it be empty?
This is the result:
.

Answer 1

axis cs (axis coordinate system) is the coordinate system to be used inside of an axis. It's the default coordinate system, so it's not necessary to prefix coordinates with it.

Therefore, command

\path[name path=axis] (axis cs:0,0) -- (axis cs:1,0);

defines a path from coordinate (0,0) to coordinate (1,0). This path is too short to intersect with the Gaussian curve which is drawn between x=0 and x=4.

The easiest solution would be to use (remember, no need for axis cs prefix)

\path[name path=axis] (0,0) -- (5,0);

Perhaps you confused axis cs with rel axis cs (relative coordinate system) which uses the complete axis vectors as units, i.e.

\path[name path=axis] (rel axis cs:0,0) -- (rel axis cs:1,0);

defines axis as the horizontal line between the lower and upper points in axis range. This is what you probably wanted to write.

\documentclass{article}
\usepackage{amsmath}
\usepackage[usenames,dvipsnames]{xcolor}
\usepackage{pgfplots}

\colorlet{shadecolor}{gray!10}
\numberwithin{equation}{section}

\pgfplotsset{%
    compat=1.11,
    standard/.style={%
        clip=false,
        mark=none,
        smooth,
        axis x line*=bottom,
        axis y line=left,
        axis x line=middle,
        axis y line=middle,
        enlarge x limits=0.15,
        enlarge y limits=0.15,
        every axis x label/.style={at={(current axis.right of origin)},anchor=north west},
        every axis y label/.style={at={(current axis.above origin)},anchor=north east}
    }
}
\usepgfplotslibrary{fillbetween}

\begin{document}
\begin{figure}[!h]
    \centering
    \begin{tikzpicture}
        \newcommand\gaussian{1.0298076 * exp(-(x-2)^2 / 0.48)}
        \pgfplotsset{ticks=none};

        \begin{axis}[standard,every axis plot post/.append style={%
                domain=0:4,samples=200},
        xlabel={$x$},
        ylabel={$p(x)$},
        enlargelimits=upper]
            \addplot[name path=f,CornflowerBlue]{\gaussian};
% Next two lines are valid. Select the one you prefer
%            \path[name path=axis] (0,0) -- (5,0);
            \path[name path=axis] (rel axis cs:0,0) -- (rel axis cs:1,0); %%%% The problem
            \addplot[
                thick,
                color=blue,
                fill=blue,
                fill opacity=0.05
            ]
            fill between[
                of=f and axis,
                soft clip={domain=1.6:2.4}
            ];
            \addplot[dashed] coordinates {(2.4,0) (2.4,0.737889)};
            \addplot[dashed] coordinates {(1.6,0) (1.6,0.737889)};
            \node at (2.4,0.001) [below] {$\mu + \sigma$};
            \node at (1.6,0.001) [below] {$\mu - \sigma$};
        \end{axis}
    \end{tikzpicture}
    \caption{Intervallo di confidenza}\label{fig:distib_norm}
\end{figure}
\end{document}