Constant plots draw lines parallel to the x-axis to connect coordinates. The discontinuous edges may be drawn or not, and marks may be placed on left or right ends.
The code is from the PGFPlots 1.10 manual: “4.5.3 Constant Plots”.
\documentclass[border=10pt]{standalone}
\usepackage{pgfplots}
\pgfplotsset{width=7cm,compat=1.8}
\begin{document}
\begin{tikzpicture}
\begin{axis}
\addplot+[const plot]
coordinates
{(0,0.1) (0.1,0.15) (0.2,0.5) (0.3,0.62)
(0.4,0.56) (0.5,0.58) (0.6,0.65) (0.7,0.6)
(0.8,0.58) (0.9,0.55) (1,0.52)};
\end{axis}
\end{tikzpicture}
\end{document}