# Set a length that will adopt tikz's global lengths

I am trying to draw a rather simple tikz picture that consists of some very acurately defined levels. My code is

\documentclass{article}

\usepackage{tikz}
\usetikzlibrary{plotmarks}

\newlength{\offset}
\setlength{\offset}{4.72983cm}

\begin{document}
\begin{tikzpicture}[y=1cm]
%axis
\draw (-3,0) -- coordinate (y axis mid) (-3,20);
%ticks
\foreach \y in {0,2,...,20}
\draw (-3,\y) -- (-3.25,\y)
node[anchor=east] {\y};
%labels
\node[rotate=90, above=0.8cm] at (y axis mid) {\textbf{Energy[MeV]}};
%input channel
\draw (-2,12.8435) -- (0,12.8435);
\node[above=0.2cm] at (-1,12.8435) {$p+{}^{19}F$};
%compound nucleus
\draw[very thick] (2,20) -- (2,0) -- (5,0) -- (5,20);
\node[below=0.1cm] at (3.5,0) {${}^{20}Ne$};
%output channel
\draw (7,\offset) -- (9,4.72983);%ground state
\draw (7,5.98765+\offset) -- (9,5.98765+4.72983);%1st state
\end{tikzpicture}
\end{document}


As you can see I have set a length called offset. The problem is that at the present point the picture is rather big. What I want is to scale it only on y axis. If I change \begin{tikzpicture}[y=1cm] to \begin{tikzpicture}[y=.6cm] I get the ideal y dimension but the levels on the output channel are not set correctly.

Is there a way to define a length in tikz that will adapt tikz's lengths? What I mean is that by default tikz's length is 1cm. When using \draw (0,0) -- (0,1); one does not specify that he's talking about cm. If the global length is changed(i.e. [y=0.6]) the previous line will have 0.6 cm length. Is something like that possible?

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# Remarks

Use a macro to store the pure value, such that tikz can scale it using it's own units.

# Implementation

\documentclass[tikz]{standalone}
\usetikzlibrary{plotmarks}
\newcommand{\offset}{4.72983}
\begin{document}
\begin{tikzpicture}[y=0.2cm]
%axis
\draw (-3,0) -- coordinate (y axis mid) (-3,20);
%ticks
\foreach \y in {0,2,...,20}
\draw (-3,\y) -- (-3.25,\y)
node[anchor=east] {\y};
%labels
\node[rotate=90, above=0.8cm] at (y axis mid) {\textbf{Energy[MeV]}};
%input channel
\draw (-2,12.8435) -- (0,12.8435);
\node[above=0.2cm] at (-1,12.8435) {$p+{}^{19}F$};
%compound nucleus
\draw[very thick] (2,20) -- (2,0) -- (5,0) -- (5,20);
\node[below=0.1cm] at (3.5,0) {${}^{20}Ne$};
%output channel
\draw (7,\offset) -- (9,4.72983);%ground state
\draw (7,5.98765+\offset) -- (9,5.98765+4.72983);%1st state
\end{tikzpicture}
\end{document}
`

# Output

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Thank you very much for your answer! That was an interesting approach! Very clever! Thank's!!! –  Thanos Dec 29 '13 at 13:05