# Trapezoidal rule with pgfplots [duplicate]

I'm looking for a way to illustrate Trapezoidal rule. I've seen this code to illustrate Riemann sums (Riemann sum with pgfplots -- can't seem to make graph look right) but I don't see how to adapt it.

\documentclass[border=5mm]{standalone}

\usepackage{pgfplots}

\pgfplotsset{compat=1.9}

\begin{document}
\begin{tikzpicture}
\begin{axis}[
xtick={0,1,3,5,7,9,11,13,15,17},ytick={0,...,1.5},
xmax=18,ymax=1.2,ymin=0,xmin=0,
enlargelimits=true,
axis lines=middle,
clip=false,
domain=0:17,
axis on top
]

\addplot [draw=red, fill=red!10, ybar interval, samples=9, domain=1:17]
{x^-1}\closedcycle;
\addplot [draw=green, fill=green!10, ybar interval, samples=9, domain=17:1]
{x^-1}\closedcycle;