我使用 pgfplots 在 LaTeX 中制作了一个图表，但是当我将方程组纳入图例时，图例长度变得太小。而且我似乎无法增加图例长度。
我的代码：
\documentclass[a4paper,11pt,titlepage]{report}
\usepackage[dvipsnames]{xcolor}
\usepackage{amsmath}
\usepackage{graphicx}
\usepackage{pgfplots}
\usepackage{tikz}
\usepackage{hyperref}
\usepackage[latin1]{inputenc}
\pgfplotsset{compat=1.12}
\usetikzlibrary{patterns, decorations.markings, arrows}
\begin{document}
\begin{tikzpicture}
\begin{axis}
[
scale=1.25,
xmin = -2, xmax=2,
ymin = -1, ymax=8,
xlabel = $x_1$,
ylabel = $f(x_1)$,
grid = both,
grid style ={line width=.1pt, draw=darkgray!10},
major grid style ={line width=.2pt, draw=darkgray!50},
axis line style = {->},
axis lines = middle,
minor tick num = 4,
enlargelimits={abs=0.5},
axis line style = {latex-latex},
samples = 100,
domain = -3:3,
legend pos = outer north east,
legend style = {nodes={align=left,anchor=west,scale=0.7}},
]
% Nonlinear function
\addplot[draw=red, thick] {x^2+2};
\addlegendentry{$f(x_1) = x^2+2$}
% f(x_1) <= 4
\addplot[draw=Aquamarine, thick, opacity=0.7, postaction={
decorate, decoration={markings,
mark = at position 4/24 with {\arrow[Aquamarine, rotate=-90]{latex}},
mark = at position 5/6 with {\arrow[Aquamarine, rotate=-90]{latex}},
}}] coordinates{(-3, 4) (3,4)};
\addlegendentry{$f(x_1) \leq 4$}
% x_1 >= -1
\addplot[draw=Aquamarine, thick, opacity=0.7, legend image post style={rotate=90, xshift=-0.3cm, yshift=-0.35cm}, postaction={
decorate, decoration={markings,
mark = at position 0.35 with {\arrow[Aquamarine, rotate=-90]{latex}},
mark = at position 0.7 with {\arrow[Aquamarine, rotate=-90]{latex}},
}}] coordinates {(-1, -2) (-1, 9)};
\addlegendentry{$x_1 \geq -1$}
% Feasible area
\filldraw[blue, pattern=north west lines, pattern color=blue] (-1,2+2) -- (-1, 1+2) -- (0, 0+2) -- (1, 1+2) -- (4/3, 2+2) -- cycle;
\addlegendimage{area legend, pattern=north west lines, pattern color=blue}
\addlegendentry{Feasible Region}
% Piecewise linear:
\addplot[draw=Black, thick, legend image post style={yshift=-0.35cm}] coordinates {(-2, 6) (-1, 3)}; %-3x-2+2
\addplot[draw=Black, thick] coordinates {(-1, 3) (0, 2)}; %-x+2
\addplot[draw=Black, thick] coordinates {(0, 2) (1, 3)}; %x+2
\addplot[draw=Black, thick] coordinates {(1, 3) (2, 6)}; %3x-2+2
\addlegendentry{$f(x_1) =
\begin{cases}
-3x_1 & \text{if } x_1 \in [-2,-1)\\
-x_1+2 & \text{if } x_1 \in [-1,0)\\
x_1+2 & \text{if } x_1 \in [0,1)\\
3x_1 & \text{if } x_1 \in [1,2]
\end{cases} $}
\end{axis}
\end{tikzpicture}
\end{document}
好像：
任何帮助将不胜感激。
答案1
感谢@samcarter_is_at_topanswers.xyz 的建议，我找到了解决方案，方法是[text depth=1cm]添加
\addlegendentry{$f(x_1) =
\begin{cases}
-3x_1 & \text{if } x_1 \in [-2,-1)\\
-x_1+2 & \text{if } x_1 \in [-1,0)\\
x_1+2 & \text{if } x_1 \in [0,1)\\
3x_1 & \text{if } x_1 \in [1,2]
\end{cases} $}
所以：
\addlegendentry[text depth=1cm]{$f(x_1) =
\begin{cases}
-3x_1 & \text{if } x_1 \in [-2,-1)\\
-x_1+2 & \text{if } x_1 \in [-1,0)\\
x_1+2 & \text{if } x_1 \in [0,1)\\
3x_1 & \text{if } x_1 \in [1,2]
\end{cases} $}
并通过legend image post style={yshift=-0.35cm}删除
\addplot[draw=Black, thick, legend image post style={yshift=-0.35cm}] coordinates {(-2, 6) (-1, 3)};
改编代码：
\documentclass[a4paper,11pt,titlepage]{report}
\usepackage[dvipsnames]{xcolor}
\usepackage{amsmath}
\usepackage{graphicx}
\usepackage{pgfplots}
\usepackage{tikz}
\usepackage{hyperref}
\usepackage[latin1]{inputenc}
\pgfplotsset{compat=1.12}
\usetikzlibrary{patterns, decorations.markings, arrows}
\begin{document}
\begin{tikzpicture}
\begin{axis}
[
scale=1.25,
xmin = -2, xmax=2,
ymin = -1, ymax=8,
xlabel = $x_1$,
ylabel = $f(x_1)$,
grid = both,
grid style ={line width=.1pt, draw=darkgray!10},
major grid style ={line width=.2pt, draw=darkgray!50},
axis line style = {->},
axis lines = middle,
minor tick num = 4,
enlargelimits={abs=0.5},
axis line style = {latex-latex},
samples = 100,
domain = -3:3,
legend pos = outer north east,
legend style = {nodes={align=left,anchor=west,scale=0.7}},
]
% Nonlinear function
\addplot[draw=red, thick] {x^2+2};
\addlegendentry{$f(x_1) = x^2+2$}
% f(x_1) <= 7
\addplot[draw=Aquamarine, thick, opacity=0.7, postaction={
decorate, decoration={markings,
mark = at position 4/24 with {\arrow[Aquamarine, rotate=-90]{latex}},
mark = at position 5/6 with {\arrow[Aquamarine, rotate=-90]{latex}},
}}] coordinates{(-3, 4) (3,4)};
\addlegendentry{$f(x_1) \leq 4$}
% x_1 >= -1
\addplot[draw=Aquamarine, thick, opacity=0.7, legend image post style={rotate=90, xshift=-0.3cm, yshift=-0.35cm}, postaction={
decorate, decoration={markings,
mark = at position 0.35 with {\arrow[Aquamarine, rotate=-90]{latex}},
mark = at position 0.7 with {\arrow[Aquamarine, rotate=-90]{latex}},
}}] coordinates {(-1, -2) (-1, 9)};
\addlegendentry{$x_1 \geq -1$}
% Feasible area
\filldraw[blue, pattern=north west lines, pattern color=blue] (-1,2+2) -- (-1, 1+2) -- (0, 0+2) -- (1, 1+2) -- (4/3, 2+2) -- cycle;
\addlegendimage{area legend, pattern=north west lines, pattern color=blue}
\addlegendentry{Feasible Region}
% Piecewise linear:
\addplot[draw=Black, thick] coordinates {(-2, 6) (-1, 3)}; %-3x-2+2
\addplot[draw=Black, thick] coordinates {(-1, 3) (0, 2)}; %-x+2
\addplot[draw=Black, thick] coordinates {(0, 2) (1, 3)}; %x+2
\addplot[draw=Black, thick] coordinates {(1, 3) (2, 6)}; %3x-2+2
\addlegendentry[text depth=1cm]{$f(x_1) =
\begin{cases}
-3x_1 & \text{if } x_1 \in [-2,-1)\\
-x_1+2 & \text{if } x_1 \in [-1,0)\\
x_1+2 & \text{if } x_1 \in [0,1)\\
3x_1 & \text{if } x_1 \in [1,2]
\end{cases} $}
\end{axis}
\end{tikzpicture}
\end{document}
结果：
