我想要画这个图：
我写了一个代码如下：
\documentclass[tikz]{standalone}
\usepackage{pgfplots}
\usepackage{pgfplotstable}
\begin{document}
\begin{tikzpicture}
\draw [<->] (0,2) node (yaxis) [above] {$y$} |- (3,0) node (xaxis) [right] {$x$};
\node[scale=0.4] at (0.3,-0.08) {$a$};
\node[scale=0.4] at (1.3,-0.1) {$\frac{a+b}{2}$};
\node[scale=0.4] at (2.3,-0.08) {$b$};
\path[purple,rotate=360,draw,fill=gray!50] (2.3,1.2) parabola (0.3,0.5);
\draw [black] (0.3,0.5) to [out=20,in=-160] (1.3,1.03) to [out=20,in=160] (2.3,1.2);
\node[scale=0.3] at (0.65,0.82) {\rotatebox{30}{$y=p_2(x)$}};
\draw[] (1.3,0) to (1.3,1.03);
\draw[] (0.3,0) to (0.3,0.5);
\draw[] (2.3,0) to (2.3,1.2);
\node[scale=0.3] at (2,1.32) {\rotatebox{4}{$y=f(x)$}};
\end{tikzpicture}
\end{document}
输出为：
我该如何纠正？另外，$x$ 和 $y$ 太大了。
答案1
您可以使用交点来修复交叉点，也可以使用 fillbetween 库来修复灰色区域。不过，由于您使用的是 pgfplots，如果您有曲线的闭式公式，那么我们就不必手动找到中点了，这样会容易得多。
\documentclass[tikz]{standalone}
\usepackage{pgfplots}
\pgfplotsset{compat=1.14}
\usepgfplotslibrary{fillbetween}
\usetikzlibrary{intersections}
\begin{document}
\begin{tikzpicture}
\begin{axis}[axis lines=center, ytick=\empty,
ymax=1.3, ymin=0, xmax=2.6,xmin=0,
xtick={0.3,1.3,2.3}, xticklabels={$a$,$\frac{a+b}{2}$,$b$},
axis line style={-}, xlabel=$x$, ylabel=$y$]
\draw[name path=A, purple] (2.3,1.2) parabola (0.3,0.5);
\node[] at (1.6,1.2){$y=f(x)$};
\path[name path=B] (\pgfkeysvalueof{/pgfplots/xmin},0)
--(\pgfkeysvalueof{/pgfplots/xmax},0);
\addplot[gray!50] fill between[of=A and B,soft clip={domain=0.3:2.3}];
\draw[name path=C] (1.3,0) -- (1.3,1.025);
\path[name intersections={of=A and C}] coordinate (midpoint) at (intersection-1);
\draw (0.3,0.5) ..controls ++(0.1,0.3) and ++(-0.2,-0.05) .. (midpoint)
node[pos=0.5,above left]{$y=p_2(x)$}
..controls ++(0.2,0.05) and ++(-0.1,-0.05) .. (2.3,1.2);
\end{axis}
\end{tikzpicture}
\end{document}
