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\addplot[red,line width=1.5pt] {mod(\x,1)<1? mod(\x,1)/5:0};

\documentclass[border=10pt]{standalone}
\usepackage{pgfplots}
\pgfplotsset{compat=1.18}
\begin{document}\begin{tikzpicture} 
	\centering
	\begin{axis}[
        %xtick=\empty,ytick=\empty,
		xlabel={$t$},
		ylabel={$Q(t)  F(t)$},
		ymax=1,
		grid=major,
		domain=0:5,
		samples=100,
        width=\textwidth,
        height = 8cm,
		axis lines=left,
        %heigth=5cm,
		legend pos=south east, % Legende oben rechts
		]
		
		\addplot[red,line width=1.5pt] {mod(\x,1)<1? mod(\x,1):0};
		\addlegendentry{$Q(t)$};
		\addplot[blue,line width=1.5pt]{0.2*x};
		\addlegendentry{$F(t)$};
		
\end{axis}

	
	\end{tikzpicture}

		\end{document}

\documentclass[border=10pt]{standalone}
\usepackage{pgfplots}
\pgfplotsset{compat=1.18}
\begin{document}
\begin{tikzpicture}
\begin{axis}[axis lines = middle,
    domain = 0:6, ymin=0, ymax=3,
    samples = 200, no marks, thick]
  \addplot {mod(\x,1)<1? mod(\x,1):0};
\end{axis}
\end{tikzpicture} 
\end{document}

\begin{tikzpicture}
\begin{axis}[
	xlabel={x},
	%ylabel={y},
	ymax=1.1,
	grid=major,
	domain=0:5,
	samples=100,
	axis lines=left,
	legend pos=south east,
	]
	
	\addplot[red,domain=0:1,line width=1.5pt] {x^2};
	\addlegendentry{2}
	\addplot[blue,domain=0:1,line width=1.5pt] {x};
	\addlegendentry{$1$}
	
\end{axis}
\end{tikzpicture}