קווי שגיאה עם pgfplots

\begin{tikzpicture}
  \begin{axis}[xlabel={Length (m)}, ylabel={Period (s)}]
    \addplot+[error bars/.cd, y dir=both, y explicit]
      coordinates {
        (0.2,0.90) +- (0,0.05)
        (0.4,1.27) +- (0,0.04)
        (0.6,1.55) +- (0,0.06)
        (0.8,1.80) +- (0,0.05)
        (1.0,2.01) +- (0,0.07)
      };
  \end{axis}
\end{tikzpicture}

\begin{tikzpicture}
  \begin{axis}[xlabel={Dose}, ylabel={Response}, ymin=0]
    \addplot+[only marks, error bars/.cd, y dir=both, y explicit]
      table[x=dose, y=mean, y error=sd] {
        dose mean sd
        1    2.1  0.3
        2    3.4  0.4
        4    5.0  0.6
        8    6.2  0.5
      };
  \end{axis}
\end{tikzpicture}

\begin{tikzpicture}
  \begin{axis}[ymin=0, legend pos=north west]
    \addplot+[error bars/.cd, y dir=plus, y fixed=0.4]
      coordinates {(1,2) (2,3) (3,3.5) (4,4.4)};
    \addplot+[error bars/.cd, y dir=both, y fixed relative=0.1]
      coordinates {(1,1) (2,1.6) (3,2.2) (4,2.5)};
    \legend{fixed $\pm 0.4$ (upper), relative 10\%}
  \end{axis}
\end{tikzpicture}

\begin{tikzpicture}
  \begin{axis}[width=7cm, height=5cm, xmin=0.5, xmax=3.5]
    \addplot+[only marks, error bars/.cd, y dir=both, y explicit]
      coordinates {
        (1,2.0) += (0,0.5) -= (0,0.1)
        (2,3.0) += (0,0.2) -= (0,0.6)
        (3,2.6) += (0,0.3) -= (0,0.3)
      };
  \end{axis}
\end{tikzpicture}

\begin{tikzpicture}
  \begin{axis}[xlabel=$x$, ylabel=$y$]
    \addplot+[only marks, mark=square*,
              error bars/.cd,
                x dir=both, x explicit,
                y dir=both, y explicit,
                error bar style={red, thick}]
      coordinates {
        (1.0,1.2) +- (0.15,0.20)
        (2.0,2.1) +- (0.20,0.15)
        (3.0,2.8) +- (0.10,0.30)
      };
  \end{axis}
\end{tikzpicture}

\documentclass{article}
\usepackage{pgfplots}
\usepgfplotslibrary{fillbetween}
\begin{document}
\begin{tikzpicture}
  \begin{axis}[domain=0:4, samples=40, xlabel=$t$, ylabel=$y$]
    \addplot[name path=upper, draw=none] {exp(-0.5*x) + 0.1 + 0.05*x};
    \addplot[name path=lower, draw=none] {exp(-0.5*x) - 0.1 - 0.05*x};
    \addplot[blue!20] fill between[of=upper and lower];
    \addplot[blue, thick] {exp(-0.5*x)};
  \end{axis}
\end{tikzpicture}
\end{document}