# Monotony table and signs

\documentclass[10pt]{article}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{pgf,tikz}
\usepackage{tkz-tab}
\usetikzlibrary{shapes,snakes,arrows,backgrounds}
\usepackage{graphicx}
\usepackage{xcolor}

\begin{document}

\begin{tikzpicture}
\tkzTabInit[color, colorT = red!30, colorC = yellow!20,colorL        =             blue!20,  colorV = lightgray!20, espcl=4]{$x$ /1,$f''(x)$ /1, $f'(x)$ /2,   $f(x)$ /2 }
{$-\infty$ ,$0$, $+\infty$}%
\tkzTabLine {,+ , z,+ ,  }
\tkzTabVar[color=red]{-/  $-\infty$,R/ /,+/ $+\infty$ /}%
\tkzTabVal[draw]{1}{3}{0.5}{}{$0$}
\tkzTabVar{+/ $+\infty$, -/$f(0)=0$/, +/ $+\infty$ / }

\end{tikzpicture}
\end{document}


The above code produces the following shape:

I want , however, to add one last thing in the table. In the box of the derivative of f, that shows its monotony I would like to add the sign of the derivative. That is , add a plus sing at the right in the center below the arrow and a minus sign at the left in the center above the arrow.

How can I do that?

The easiest way is to add these two lines after \tkzTabVal in your code:
\tkzTabVal{1}{3}{.25}{}{$-$}
\tkzTabVal{1}{3}{.75}{}{$+$}