Fantastico!!! Questo è il codice che ho usato per risolvere il problema:
`\documentclass{article}
\usepackage{tikz,pgfplots}
\usetikzlibrary{calc,backgrounds}
\begin{document}
\begin{tikzpicture}[font=\small]
\begin{axis} [width=\textwidth,
axis lines=middle,
xmin=-20,xmax=600,ymin=-5000,ymax=4000,
enlargelimits,
xtick=\empty,
ytick=\empty,
xlabel=$x$,
ylabel=$y$,grid]
\begin{pgfonlayer}{background}
\addplot [domain=100:400, samples=200,smooth,draw=none,fill=blue!50!cyan!20] {-0.1*x*x+60*x-5000} –(axis cs:400,0);
\end{pgfonlayer}
\begin{pgfonlayer}{background}
\addplot [domain=0:100, samples=200,smooth,draw=none,fill=red!50!orange!20] {-0.1*x*x+60*x-5000} — (axis cs:0,0);
\end{pgfonlayer}
\addplot [domain=0:400, samples=200,smooth,very thick,blue] {-0.1*x*x+60*x-5000};
\addplot [domain=400:600, samples=200,smooth,very thick,dashed] {-0.1*x*x+60*x-5000};
\draw (axis cs:400,-10000) — (axis cs:400,10000);
\node [below] at (axis cs:400,0) {$400$};
\node (a1) at (axis cs:550,3500) {Zona di utile};
\node (b1) at (axis cs:200,-4000) {Zona di perdita};
\node (a2) at (axis cs:350,2000) {};
\node (b2) at (axis cs:30,-1500) {};
\draw [->] (a1.south) — (a2.center);
\draw [->] (b1.north) — (b2.center);
\end{axis}
\end{tikzpicture}
\end{document}`
Grazie milla, Claudio, per il tuo preziosissimo aiuto!!! 🙂 🙂 🙂