# How to graph a logistic equation using tikz

I am not sure why this doesn't work. At first I thought the issue was x values close to zero. So I broke it up into two separate graphs that ignored x values close to zero. I have tried copying code snippets form others. Theirs works (using different function to graph), but mine does not.

The error I get is:! PGF Math Error: p' orp(' in {2500*exp(0.05*-10.0)/(100+25*(exp(0.05*-10.0)-1))}')

\documentclass{standalone}

\usepackage{tikz}

\begin{document}

\begin{center}
\begin{tikzpicture}[scale=0.1]
\draw [<->,very thick] (0,-10) -- (0,100) node [above] {$y$};
\draw [<->,very thick] (-10,0) -- (160,0) node [right] {$x$};
\draw [domain=-10:160,samples=1000] plot (\x,{2500*exp(0.05*\x)/(100+25*(exp(0.05*\x)-1))});
\end{tikzpicture}
\end{center}


It is because the dimension is too large to plot. A work around is to rewrite the function to be plotted in a way that it won't cause dimension too large error.

\documentclass[border=3mm]{standalone}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}[scale=0.1]
\draw [<->,very thick] (0,-10) -- (0,100) node [above] {$y$};
\draw [<->,very thick] (-10,0) -- (160,0) node [right] {$x$};
\draw [domain=-10:160,samples=100] plot (\x,{100/(1+3*(exp(-0.05*\x))});
\end{tikzpicture}
\end{document}


• Thank you so much...this works great. I had fun reproducing the math as well. – Garth Fleming Dec 11 '18 at 7:03

For comparism purpose. Run with xelatex

\documentclass[pstricks]{standalone}
\usepackage{pst-plot}
\begin{document}

\begin{psgraph}[Dx=25,Dy=25,llx=-0.5,lly=-0.5,
urx=0.5,ury=0.5,xAxisLabel=$x$,
yAxisLabel=$y$]{->}(0,0)(-15,-10)(160,110){10cm}{8cm}
\psplot[plotpoints=1000,algebraic,linewidth=1.5pt,
linecolor=blue]{-10}{160}{2500*Euler^(0.05*x)/(100+25*(Euler^(0.05*x)-1))}
\end{psgraph}

\end{document}