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”.

Constant plot

Edit and compile if you like:
    \addplot+[const plot]
        {(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)};
Click to download: constant-plot.texconstant-plot.pdf