\documentclass{standalone}

\usepackage{tikz,pgfplots}
\usepackage{siunitx}
\usepackage[siunitx,european]{circuitikz}
\usepackage{mathrsfs}
\usetikzlibrary{arrows}
\usepackage{amsbsy}
\usetikzlibrary{fadings}
\usetikzlibrary{patterns}
\usetikzlibrary{automata,positioning}
\usepackage{mwe,subcaption,tikz}
\tikzset{boximg/.style={remember picture,red,thick,draw,inner sep=0pt,outer sep=0pt}}
\pgfplotsset{compat=1.11}
\begin{document}
\definecolor{mygreen}{rgb}{0,0.78,0}
\begin{tikzpicture}

	\draw[<->, thick] (0,3) node[above]{$\frac{U_{out}}{U_{in}}\cdot N$} |- (5.25,0) node[right]{$D$};
	\draw[blue, thick, rounded corners=10pt] (0,1) -- (0.5,1.111) -- (1,1.25) -- (1.5, 1.429) -- (2,1.667) -- (2.5,2);
	\draw[blue, thick, rounded corners=10pt, dashed, dash phase=6] (3.5,3.333) -- (3,2.5) -- (2.5,2);
	\draw(1.25,0.1) -- (1.25,-0.1) node[below]{0.25}
	(2.5,0.1) -- (2.5,-0.1) node[below]{0.50}
	(3.75,0.1) -- (3.75,-0.1) node[below]{0.75}
	(5,0.1) -- (5,-0.1) node[below]{1.00};
\end{tikzpicture}
\end{document}