\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] {68.3\%};
- \draw (-2,0.25) -- (2,0.25) node [midway, fill=white] {95.5\%};
- \draw (-3,0.15) -- (3,0.15) node [midway, fill=white] {99.7\%};
+ \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);