Using `\input` to insert a section of a document into another

You could use the catchfilebetweentags package.

In your main file:

\usepackage{catchfilebetweentags}
....
\ExecuteMetaData[file.tex]{graph}

and in your file.tex

%<*graph>
code for the graph
%</graph>

You can set up your file containing the code for the graphs like

% from https://tex.stackexchange.com/a/234521/4427

\GRAPH graph1
\begin{tikzpicture}
  \begin{axis}[ylabel={Y label},xmin=-1,xmax=1,width=3in,height=2in]
    \addplot coordinates {(-1,-1) (1,1)};
    \coordinate (NE) at (rel axis cs: 1,1);% upper right corner of axis
  \end{axis}
  \path (-15mm,-5mm) ($(NE)+(3mm,2mm)$);
  %\draw[red] (-15mm,-5mm) rectangle ($(NE)+(3mm,2mm)$);% to fine tune offsets
\end{tikzpicture}
\ENDGRAPH

\GRAPH graph2
\begin{tikzpicture}
  \begin{axis}[xmin=-10,xmax=10,width=3in,height=2in]
    \addplot coordinates {(-10,-10) (10,10)};
    \coordinate (NE) at (rel axis cs: 1,1);
  \end{axis}
  \path (-15mm,-5mm) ($(NE)+(3mm,2mm)$);
  %\draw[red] (-15mm,-5mm) rectangle ($(NE)+(3mm,2mm)$);% to fine tune offsets
\end{tikzpicture}
\ENDGRAPH

(the first comment is just to show comments are honored and to state the source for the code). I saved it as murfitt-graphs.tex, but the name is arbitrary and you can have several files like this. The structure is important: the line \GRAPH should be followed by the symbolic name of the graph (whatever string of ASCII characters you want).

Now your main document should have the following code; the relevant part is between %% define \inputgraph and `%% end

\documentclass{article}

\usepackage{pgfplots}
\usetikzlibrary{calc}

%% define \inputgraph
\newcommand{\inputgraph}[2]{% #1 = file, #2 = graph name
  \long\def\GRAPH ##1#2 {}%
  \input{#1}
}
\let\ENDGRAPH\endinput
%% end


\begin{document}

\inputgraph{murfitt-graphs}{graph2}

\inputgraph{murfitt-graphs}{graph1}

\inputgraph{murfitt-graphs}{graph2}

\end{document}

What's the advantage over catchfilebetweentags? That here the code is not read in as the argument of a macro. In case you use a wrong label, you'll get an error

Runaway argument?
graph1 \begin {tikzpicture} \begin {axis}[ylabel={Y label},xmin=-1,xm\ETC.
! File ended while scanning use of \GRAPH.

and no graph will be processed.

With the clipboard package:

graphs.tex:

\documentclass{article}
\usepackage{clipboard}
\newclipboard{mygraphs}
\begin{document}
 Something ...
\Copy{graph1}{Code of graph 1}
\Copy{graph2}{Code of graph 2}
\Copy{graph1}{Code of graph 3}
 More code ... 
\end{document}

main.tex:

\documentclass{article}
\usepackage{clipboard}
\openclipboard{mygraphs}
\begin{document}
A nice graph:\Paste{graph2}
\end{document}

You must run pdflatex in the document with the \Copy first and then compile the document/s with \Paste commands.