在科研論文寫作中，我們經常需要放大局部圖片來顯示細節，即繪制圖中圖。在Matlab中可以使用magnify或則axes函數，網上有很多例子，這里不再贅述。本文主要講解如何使用tikz/pgfplots來畫局部放大圖中圖。

繪制局部放大圖，需要使用到spy宏包，本文主要參考pgfplots手冊。下面給出一個最簡單的實例，具體代碼如下：

\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}

結果如下：

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代碼簡單易懂，也給出了部分注釋。為了使得更加個性化，我們可以修改放大圖的輪廓，或則背景色，具體代碼如下：

\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}

顯示效果如下：

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由上圖可以看到，相比于Matlab，這里局部放大圖更加靈活和個性化。但是Matlab的局部放大圖一般都有坐標軸，方便查看局部圖的值大小。在Tikz/pgfplots中利用Spy宏包實現相同的效果比較困難。但是我們可以另辟蹊徑: 只需要利用node，在node中重新畫一個圖，只是自己設置橫縱坐標的值，從而實現局部放大。具體代碼實現如下：

\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}

結果如下：

總結

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