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: “4.5.1 Linear Plots”.

Linear plot

\documentclass[border=10pt]{standalone}
\usepackage{pgfplots}
\pgfplotsset{width=7cm,compat=1.10}
\begin{document}
\begin{tikzpicture}
  \begin{axis}
    \addplot+[sharp plot] coordinates
      {(0,0) (1,2) (2,3)};
  \end{axis}
\end{tikzpicture}
\end{document}
Click to download: linear-plot.texlinear-plot.pdf