弯曲的需求曲线

Question

\documentclass{article}
\usepackage{pgfplots}
\pgfplotsset{compat=1.18}

\begin{document}

\begin{tikzpicture}
\begin{axis}[
    axis lines = left,
    xlabel = $Q$,
    ylabel = {$P$},
    xmin=0, xmax=30,
    ymin=0, ymax=30,
    xtick={0,5,...,25},
    ytick={0,5,...,25},
]
\addplot [
    domain=0:20, 
    color=red,
]
{20-0.5*x};
\addplot [
    domain=0:25, 
    color=blue,
    dashed,
]
{10-0.4*x};
\addplot [
    domain=0:25, 
    color=green,
    dashed,
]
{5-0.2*x};
\addplot [
    domain=0:25, 
    color=black,
]
{15-0.6*x};
\addplot [
    domain=0:25, 
    color=black,
]
{10-0.4*x};
\addplot [
    domain=0:25, 
    color=black,
]
{5-0.2*x};
\node[label={180:{$P_1$}},circle,fill,inner sep=2pt] at (axis cs:10,15) {};
\node[label={180:{$P_2$}},circle,fill,inner sep=2pt] at (axis cs:20,10) {};
\node[label={180:{$P_3$}},circle,fill,inner sep=2pt] at (axis cs:25,5) {};
\node[label={0:{$Q_1$}},circle,fill,inner sep=2pt] at (axis cs:10,0) {};
\node[label={0:{$Q_2$}},circle,fill,inner sep=2pt] at (axis cs:20,0) {};
\node[label={0:{$Q_3$}},circle,fill,inner sep=2pt] at (axis cs:25,0) {};
\legend{$Q_1=200-10P$,$Q_2=100-4P$,$MR_1$,$MR_2$,$D$,$MR$}
\end{axis}
\end{tikzpicture}