\pgfmathdeclarefunction{gauss}{2}{%
  \pgfmathparse{1/(#2*sqrt(2*pi))*exp(-((x-#1)^2)/(2*#2^2))}%
}

\pgfkeys{/pgf/decoration/.cd,
  distance/.initial=10pt
}

\pgfdeclaredecoration{add dim}{final}{
  \state{final}{% 
    \pgfmathsetmacro{\dist}{5pt*\pgfkeysvalueof{/pgf/decoration/distance}/abs(\pgfkeysvalueof{/pgf/decoration/distance})}    
    \pgfpathmoveto{\pgfpoint{0pt}{0pt}}             
    \pgfpathlineto{\pgfpoint{0pt}{2*\dist}}   
    \pgfpathmoveto{\pgfpoint{\pgfdecoratedpathlength}{0pt}} 
    \pgfpathlineto{\pgfpoint{(\pgfdecoratedpathlength}{2*\dist}}     
    \pgfsetarrowsstart{latex}
    \pgfsetarrowsend{latex}
    \pgfpathmoveto{\pgfpoint{0pt}{\dist}}
    \pgfpathlineto{\pgfpoint{\pgfdecoratedpathlength}{\dist}} 
    \pgfusepath{stroke} 
    \pgfpathmoveto{\pgfpoint{0pt}{0pt}}
    \pgfpathlineto{\pgfpoint{\pgfdecoratedpathlength}{0pt}}
}}

\tikzset{dim/.style args={#1,#2}{decoration={add dim,distance=#2},
  decorate,
  postaction={decorate,decoration={text along path,
  raise=#2,
  text align={align=center},
  text={#1}}}}
}

\begin{tikzpicture}
  \begin{axis}[every axis plot post/.style={
      mark=none,domain=-3:3,samples=50,smooth}, 
    axis x line=bottom, 
    axis y line=left,
    enlargelimits=upper,
    x=\textwidth/10,
    ytick={0.55},
    yticklabels={\(\frac{1}{\sigma \sqrt{2\pi}}\)}, 
    xtick={-2,-1,0,1,2},
    x tick label style = {font=\footnotesize},
    xticklabels={\((\mu-2\sigma)\), \((\mu-\sigma)\), \(\mu\), \((\mu+\sigma)\), \((\mu+2\sigma)\)},
    xlabel={\(x\)},
    every axis x label/.style={at={(current axis.right of origin)},anchor=north west},
    every axis y label/.style={at={(axis description cs:-0.02,0.2)}, anchor=south west, rotate=90},
    ylabel={\(\Pr(X=x)\)}]
    \addplot {gauss(0,0.75)};
    \fill[red!30] (-3,0)  -- plot[id=f3,domain=-3:3,samples=50] function {1/(0.75*sqrt(2*pi))*exp(-((x)^2)/(2*0.75^2))} -- (3,0) -- cycle;
    \fill[darkgray!30] (3,0)  -- plot[id=f3,domain=-3:3,samples=50] function {1/(0.75*sqrt(2*pi))*exp(-x*x*0.5/(0.75*0.75))} -- (3,0) -- cycle;
    \fill[lightgray!30] (-2,0)  -- plot[id=f3,domain=-2:2,samples=50] function {1/(0.75*sqrt(2*pi))*exp(-x*x*0.5/(0.75*0.75))} -- (2,0) -- cycle;
    \fill[white!30] (-1,0)  -- plot[id=f3,domain=-1:1,samples=50] function {1/(0.75*sqrt(2*pi))*exp(-x*x*0.5/(0.75*0.75))} -- (1,0) -- cycle;
    \begin{scope}[<->]
      \draw (-1,0.35) -- (1,0.35) node [midway, fill=white] {68.27\%};
      \draw (-2,0.25) -- (2,0.25) node [midway, fill=white] {95.35\%};
      \draw (-3,0.15) -- (3,0.15) node [midway, fill=white] {99.73\%};
    \end{scope}
    \begin{scope}[-, dashed, gray]
      \draw (-1,0) -- (-1, 0.35);
      \draw (1,0) -- (1, 0.35);
      \draw (-2,0) -- (-2, 0.25);
      \draw (2,0) -- (2, 0.25);
      \draw (-3,0) -- (-3, 0.15);
      \draw (3,0) -- (3, 0.15);
    \end{scope}
  \end{axis}
  \begin{axis}[every axis plot post/.append style={
      mark=none,domain=-3:3,samples=50,smooth}, 
    axis x line=bottom, 
    enlargelimits=upper,
    x=\textwidth/10,
    xtick={-2,-1,0,1,2},
    axis x line shift=30pt,
    hide y axis,
    x tick label style = {font=\footnotesize},
    xlabel={\(Z\)},
    every axis x label/.style={at={(axis description cs:1,-0.25)},anchor=south west}]
    \addplot {gauss(0,0.75)};
  \end{axis}
\end{tikzpicture}
