Addlegendentry 无法正确显示图例中的颜色 PGFPlots

Question

请考虑以下 MWE：

\documentclass[11pt,twoside]{article}
\usepackage[spanish]{babel}
\usepackage[utf8]{inputenc}
\usepackage{pgfplots}
\usetikzlibrary{arrows.meta, patterns}
\pgfplotsset{compat=1.8}
\usepackage{amsmath}

\pgfplotsset{
  Cus/.style={
        axis equal image,
        axis lines = center,
        xlabel = $x$,
        ylabel = $y$
    }
}
\begin{document}
\begin{tikzpicture}
    \pgfplotsset{ticks=none}
    \begin{axis}[
        Cus,
        xmin=-2.1, xmax=4.1,
        legend pos=outer north east,
        legend cell align={left}
        ]
        \addplot[very thick,blue,samples=300,domain=1:4.1] {sqrt((x-1)/2)};
        \addplot[very thick,blue,samples=300,domain=1:4.1] {-sqrt((x-1)/2)};
        \addlegendentry{$x-2y^2=1$}
        \addplot[very thick,red,samples=300,domain=1:4.1] {sqrt(x-1)};
        \addplot[very thick,red,samples=300,domain=1:4.1] {-sqrt(x-1)};
        \addlegendentry{$x-y^2=1$}
    \end{axis}
\end{tikzpicture}
\end{document}

我做错了什么？如何让第二个图例具有相应的颜色（红色）？

谢谢！

Answer 1

发生这种情况的原因是，您添加了四个图，但只添加了两个图例条目。这些条目对应于前两个图，它们都是蓝色的。尝试添加另一个图例条目；它将是红色的。

解决这个问题的简单方法是用 画出上部部分\addlegendentry，然后用 画出下部部分\addlegendentry。

\documentclass[11pt,twoside]{article}
\usepackage[spanish]{babel}
\usepackage[utf8]{inputenc}
\usepackage{pgfplots}
\usetikzlibrary{arrows.meta, patterns}
\pgfplotsset{compat=1.8}
\usepackage{amsmath}

\pgfplotsset{
  Cus/.style={
        axis equal image,
        axis lines = center,
        xlabel = $x$,
        ylabel = $y$
    }
}
\begin{document}
\begin{tikzpicture}
    \pgfplotsset{ticks=none}
    \begin{axis}[
        Cus,
        xmin=-2.1, xmax=4.1,
        legend pos=outer north east,
        legend cell align={left}
        ]
        \addplot[very thick,blue,samples=300,domain=1:4.1] {-sqrt((x-1)/2)};
        \addlegendentry{$x-2y^2=1$}
        \addplot[very thick,red,samples=300,domain=1:4.1] {-sqrt(x-1)};
        \addlegendentry{$x-y^2=1$}
        \addplot[very thick,blue,samples=300,domain=1:4.1] {sqrt((x-1)/2)};
        \addplot[very thick,red,samples=300,domain=1:4.1] {sqrt(x-1)};
    \end{axis}
\end{tikzpicture}
\end{document}

Answer 2

为了避免添加“虚拟”图例条目，您可以使用forget plot或 - 在给定的示例中 - 一次性以参数方式绘制图。

有关详细信息，请查看代码中的注释。

% used PGFPlots v1.15
\documentclass[border=5pt]{standalone}
\usepackage{pgfplots}
    \pgfplotsset{
        Cus/.style={
            axis equal image,
            axis lines=center,
            xlabel=$x$,
            ylabel=$y$,
        },
    }
\begin{document}
\begin{tikzpicture}
    \begin{axis}[
        Cus,
        xmin=-2.1,
        xmax=4.1,
        legend pos=outer north east,
        legend cell align={left},
        ticks=none,
        % moved common `\addplot' options here
        no markers,
        smooth,
        domain=1:4.1,
        samples=101,
        every axis plot/.append style={
            very thick,
        },
    ]
        % either add `forget plot' to each first plot half, which avoids
        % increasing the `cycle list' and avoids counting for the legend ...
        \addplot+ [forget plot] {sqrt((x-1)/2)};
        \addplot                {-sqrt((x-1)/2)};
            \addlegendentry{$x-2y^2=1$}
%        \addplot+ [forget plot] {sqrt(x-1)};
%        \addplot                {-sqrt(x-1)};
        % ... or draw the lines as parametric plots which doesn't need that
        % much `samples' and can be drawn in one go
        \addplot+ [domain=-2:2,samples=25] (x^2 + 1,x);
            \addlegendentry{$x-y^2=1$}
    \end{axis}
\end{tikzpicture}
\end{document}