This image is two tikzpictures side by side, each with a newsprint background (thanks to Background image for pgfplots chart).
I want them combined, so they appear on the same torn sheet of newsprint. The title should be centered there, too.
I've looked at answers to several questions about nesting tikzpictures but couldn't solve my problem.
The not quite minimal MWE follows. (Sorry for the details and the duplicated code. I thought it best to provide the actual plots I plan to use.)

\documentclass{standalone}
\usepackage{tikz,pgfplots}
\usepackage{pgfplotstable}
\usetikzlibrary{calc,decorations.pathmorphing}
%deal with warning message in log
\pgfplotsset{compat=1.8}
\usepackage{helvet}
\usepackage[eulergreek]{sansmath}
\newcommand{\images}{../images}
\begin{document}
\pgfdeclarelayer{background}
\pgfsetlayers{background,main}
\pgfplotstableread[col sep=comma]{
year, return, assets
2007, 23, 34.9
2008, 8.6, 37
2009, -27.3, 26
2010, 11, 27.4
2011, 21.4, 32
}\endowment
\newcommand{\plotsize}{width=7cm, height=6cm}
\tikzset{
pencildraw/.style={ %
decorate,
decoration={random steps,segment length=6pt,amplitude=3pt}
},
font=\sffamily\sansmath
}
\begin{tikzpicture}
\path[clip,pencildraw] (-1.8,-1) rectangle (6,5.5);
\begin{axis}[
title={\large Harvard Endowment},
\plotsize,
ylabel={Return},
axis y line*=left,
axis x line*=middle,
xtick=data,
xticklabel style={align=center},
xticklabels={2007, 2008, 2009, 2010, 2011},
yticklabel=\pgfmathprintnumber{\tick}\,$\%$,
nodes near coords={\pgfmathprintnumber{\pgfplotspointmeta}\%},
]
\addplot table[x=year, y=return] {\endowment};
\begin{pgfonlayer}{background}
\node{\includegraphics{\images/newsprint}};
\end{pgfonlayer}
\end{axis}
\end{tikzpicture}
\begin{tikzpicture}
\path[clip,pencildraw] (-1.8,-1) rectangle (6,5.5);
\begin{axis}[
ymin=0,
ylabel={Assets},
\plotsize,
axis y line*=left,
axis x line*=bottom,
xtick=data,
xticklabel style={align=center},
xticklabels={2007, 2008, 2009, 2010, 2011},
yticklabel={\$\pgfmathprintnumber{\tick}b},
nodes near coords={\$\pgfmathprintnumber{\pgfplotspointmeta}b},
]
\addplot table[x=year, y=assets] {\endowment};
\begin{pgfonlayer}{background}
\node{\includegraphics{\images/newsprint}};
\end{pgfonlayer}
\end{axis}
\end{tikzpicture}
\end{document}

axisenvironments in onetikzpicture. Check section 4.18 “Alignment Options” (pp. 280ff.) in thepgfplotsmanual. – Qrrbrbirlbel Jun 21 '13 at 02:17