在科研论文写作中，我们经常需要放大局部图片来显示细节，即绘制图中图。在Matlab中可以使用magnify或则axes函数，网上有很多例子，这里不再赘述。本文主要讲解如何使用tikz/pgfplots来画局部放大图中图。
绘制局部放大图，需要使用到spy宏包，本文主要参考pgfplots手册。下面给出一个最简单的实例，具体代码如下：
\documentclass[10pt, final, journal, twocolumn, oneside]{IEEEtran}
%!TEX program = xelatex
% !TEX encoding = UTF-8 (utf8)
%!TEX spellcheck
\def\pgfsysdriver{pgfsys-dvipdfmx.def}
\usepackage{tikz}
\usepackage{pgfplots}
\usetikzlibrary{spy} %
\pgfplotsset{width=7cm,compat=1.14}
\begin{document}
\begin{tikzpicture}[spy using outlines= {circle, magnification=6, connect spies}]
% define the shape of spy: circle or rectangle
\begin{axis}[no markers,grid=major, every axis plot post/.append style={thick}]
\addplot coordinates {(0, 0) (0, 0.9) (1, 0.9) (2, 1) (3, 0.9) (80, 0)};
\addplot+ [line join=round] coordinates {(0, 0) (0, 0.9) (2, 0.9) (3, 1) (4, 0.9) (80, 0)};
\addplot+ [line join=bevel] coordinates {(0, 0) (0, 0.9) (3, 0.9) (4, 1) (5, 0.9) (80, 0)};
\addplot+ [miter limit=5] coordinates {(0, 0) (0, 0.9) (4, 0.9) (5, 1) (6, 0.9) (80, 0)};
\coordinate (spypoint) at (3,1);% The point to be magnified
\coordinate (magnifyglass) at (60,0.7);% The point where to see
\end{axis}
\spy [blue, size=2.5cm] on (spypoint) in node[fill=white] at (magnifyglass);
\end{tikzpicture}
\end{document}
结果如下：
代码简单易懂，也给出了部分注释。为了使得更加个性化，我们可以修改放大图的轮廓，或则背景色，具体代码如下：
\documentclass[10pt, final, journal, twocolumn, oneside]{IEEEtran}
%!TEX program = xelatex
% !TEX encoding = UTF-8 (utf8)
%!TEX spellcheck
\def\pgfsysdriver{pgfsys-dvipdfmx.def}
\usepackage{tikz}
\usepackage{pgfplots}
\usetikzlibrary{spy} %
\pgfplotsset{width=7cm,compat=1.14}
\begin{document}
\begin{tikzpicture}[spy using overlays= {rectangle, magnification=6, connect spies}]
% define the shape of spy: circle or rectangle
\begin{axis}[no markers,grid=major, every axis plot post/.append style={thick}]
\addplot coordinates {(0, 0) (0, 0.9) (1, 0.9) (2, 1) (3, 0.9) (80, 0)};
\addplot+ [line join=round] coordinates {(0, 0) (0, 0.9) (2, 0.9) (3, 1) (4, 0.9) (80, 0)};
\addplot+ [line join=bevel] coordinates {(0, 0) (0, 0.9) (3, 0.9) (4, 1) (5, 0.9) (80, 0)};
\addplot+ [miter limit=5] coordinates {(0, 0) (0, 0.9) (4, 0.9) (5, 1) (6, 0.9) (80, 0)};
\coordinate (spypoint) at (3,1);% The point to be magnified
\coordinate (magnifyglass) at (60,0.7);% The point where to see
\end{axis}
\spy [green,size=2.5cm] on (spypoint) in node at (magnifyglass);
\end{tikzpicture}
\end{document}
显示效果如下：
由上图可以看到，相比于Matlab，这里局部放大图更加灵活和个性化。但是Matlab的局部放大图一般都有坐标轴，方便查看局部图的值大小。在Tikz/pgfplots中利用Spy宏包实现相同的效果比较困难。但是我们可以另辟蹊径: 只需要利用node，在node中重新画一个图，只是自己设置横纵坐标的值，从而实现局部放大。具体代码实现如下：
\documentclass[10pt, final, journal, twocolumn, oneside]{IEEEtran}
%!TEX program = xelatex
% !TEX encoding = UTF-8 (utf8)
%!TEX spellcheck
\def\pgfsysdriver{pgfsys-dvipdfmx.def}
\usepackage{tikz}
\usepackage{pgfplots}
\usetikzlibrary{spy} %
\pgfplotsset{width=7cm,compat=1.14}
\begin{document}
\begin{tikzpicture}[pin distance=1.5cm]
\begin{axis}[no markers,grid=major, every axis plot post/.append style={thick}]
\addplot coordinates {(0, 0) (0, 0.9) (1, 0.9) (2, 1) (3, 0.9) (80, 0)};
\addplot+ [line join=round] coordinates {(0, 0) (0, 0.9) (2, 0.9) (3, 1) (4, 0.9) (80, 0)};
\addplot+ [line join=bevel] coordinates {(0, 0) (0, 0.9) (3, 0.9) (4, 1) (5, 0.9) (80, 0)};
\addplot+ [miter limit=5] coordinates {(0, 0) (0, 0.9) (4, 0.9) (5, 1) (6, 0.9) (80, 0)};
\coordinate (spy_point) at (axis cs:0,1);
\end{axis}
%plot the magnified figure using a node
\node[pin=-5:{%
\begin{tikzpicture}
\begin{axis}[no markers, thick,scale=0.8,
tiny,
xlabel={x},
ylabel={y},
xmin=0,xmax=8,
ymin=0.85,ymax=1.05,
enlargelimits,
]
\addplot coordinates {(0, 0) (0, 0.9) (1, 0.9) (2, 1) (3, 0.9) (80, 0)};
\addplot+ [line join=round] coordinates {(0, 0) (0, 0.9) (2, 0.9) (3, 1) (4, 0.9) (80, 0)};
\addplot+ [line join=bevel] coordinates {(0, 0) (0, 0.9) (3, 0.9) (4, 1) (5, 0.9) (80, 0)};
\addplot+ [miter limit=5] coordinates {(0, 0) (0, 0.9) (4, 0.9) (5, 1) (6, 0.9) (80, 0)};
\end{axis}
\end{tikzpicture}%
}] at (spy_point) {};
\end{tikzpicture}
结果如下：
