I would like to write an a in the rectangle that I marked below. I suppose the solution uses nodes somehow but until now I could not figure it out.Thanks in advance!
\documentclass{article}\usepackage{tikz}\usetikzlibrary{decorations.pathreplacing}\begin{document}\begin{tikzpicture}[scale=.35]\footnotesize \pgfmathsetmacro{\xone}{0} \pgfmathsetmacro{\xtwo}{20} \pgfmathsetmacro{\yone}{0} \pgfmathsetmacro{\ytwo}{5}\begin{scope}<+->;% grid \draw[step=1cm,gray,very thin] (\xone,\yone) grid (\xtwo,\ytwo);% ticks \foreach \x/\xtext in { 5/q_1, 6/q_2, 7/q_3, 8/q_4, 9/q_5, 10/q_6} % xKoordinate / Beschriftung \draw[gray,xshift=\x cm] (0,-.3) -- (0,0) node[below] {$\xtext$};\end{scope}% function\begin{scope}[thick,red]\filldraw[thin,red,opacity=.3] (0,0) rectangle (6,1);\filldraw[thin,red,opacity=.3] (0,1) rectangle (10,2);\filldraw[thin,blue,opacity=.3] (0,2) rectangle (3,3);\filldraw[thin,blue,opacity=.3] (0,3) rectangle (5,4);\filldraw[thin,blue,opacity=.3] (0,4) rectangle (4,5);\draw[thick, black] (5,0) -- (5,5);\draw [decorate,decoration={brace,amplitude=5pt},xshift=-4pt,yshift=0pt](0,2) -- (0,5) node [black,midway,xshift=-0.6cm] {\footnotesize $\bar{x}$};\draw [decorate,decoration={brace,amplitude=5pt},xshift=-4pt,yshift=0pt] %Amplitude für wie weit nach rechts gezogen(0,0) -- (0,2) node [black,midway,xshift=-0.6cm] {\footnotesize $\bar{y}$};\end{scope}\end{tikzpicture}\end{document}