这样的图形如何绘制？

Question

我怎样才能用 tikz 绘制这样的图形？

函数：f(x) = x^2 + 0.25、f(x) = x^2 + 0.1 和 f(x) = x^2 + 0.4

它们的对角线均为 y=x。

我开始于：

\usepackage{pgfplots}
\begin{document}
    \begin{tikzpicture}
        \begin{axis}[
            domain = -.6:1,
            samples = 50,
            axis x line = center,
            axis y line = center,
            xlabel = {$x$},
            ylabel = {$y$},
            ticks = none
            ]
            \addplot[blue] {x*x + 0.25};
            \addplot[black] {x}
        \end{axis}
    \end{tikzpicture}
\end{document}

但我不知道我错过了什么

Answer 1

您已经非常接近了，只缺少几行。更改函数以显示其他两个图很容易。

\documentclass{article}
\usepackage{tikz}
\usepackage{pgfplots}
\begin{document}
    \begin{tikzpicture}
        \begin{axis}[
            domain = -.6:1,
            samples = 50,
            axis x line = center,
            axis y line = center,
            xlabel = {$x$},
            ylabel = {$y$},
            try min ticks = 10,
            scale only axis,
            ]
            \addplot[blue] {x*x + 0.25};
            \addplot[black] {x};
        \end{axis}
    \end{tikzpicture}
\end{document}

Answer 2

如果我可以在@bmv 的答案上添加一个小樱桃，你可能会受益于像这样声明你的函数

\documentclass{article}
\usepackage{tikz}
\usepackage{pgfplots}
\begin{document}

\begin{tikzpicture}
    
    % Declare the functions you need
    \tikzset{
        declare function={
            f(\x)=\x*\x + 0.25;
            g(\x)= f(\x) - 0.15;
            h(\x)= f(\x) + 0.15;
        }
    }
    
    \begin{axis}[
            domain = -.6:1,
            samples = 50,
            axis x line = center,
            axis y line = center,
            xlabel = {$x$},
            ylabel = {$y$},
            try min ticks = 10,
            scale only axis,
        ]
        % easy to resuse
        \addplot[blue] {f(x)};
        % \addplot[red] {g(x)};
        % \addplot[green] {h(x)};
        
        \addplot[black] {x};
    \end{axis}
\end{tikzpicture}
\end{document}