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: “4.5.2 Smooth Plots”.

Smooth plot

Edit and compile if you like:
    \addplot+[smooth] coordinates
      {(0,0) (1,2) (2,3)};
Click to download: smooth-plot.texsmooth-plot.pdf