TikZ won't show the good curve?

Question

Hello wonderful people,

TikZ doesn't seem to display the function it should be, I might be missing a dumb point, but here's the problem:

First, let's take a look at this marvellous function: polynomial, arousing curves, cuts the x-axis in (almost) integer points, this function just seems too good to be true.

Now, look at this smooth code :

\documentclass{article}

\usepackage{tikz}

\begin{document}

\begin{tikzpicture}
  \draw [gray,dotted] (-6,-5) grid (5,5) ;
  \draw[->,>=latex] (-6,0) -- (5,0) node[below]{$x$} ;
  \draw[->,>=latex] (0,-5) -- (0,5) node[left]{$y$} ;

  \draw [domain=-5:4,samples=200] plot (\x,-0.0012*\x^5-0.0131*\x^4+0.0893*\x^3+0.3988*\x^2-0.5881*\x-0.8857) ;
\end{tikzpicture}

\end{document}

simple, efficient, should do the work and show us this gorgeous function we've been talking about for minutes now.

But, alas! Look at the disastrous result it produces:

Does anyone know why TikZ would want to crush my dreams of happiness like this? A "funny" thing is that it seems to be ok with the positive values of x(?)

Thanks a lot!

Answer 1

I am not quite sure why, but for some reason the even powers seem to be acting weird. When using the function as follows, the graph looks fine:

\begin{tikzpicture}
  \draw [gray,dotted] (-6,-5) grid (5,5) ;
  \draw[->,>=latex] (-6,0) -- (5,0) node[below]{$x$} ;
  \draw[->,>=latex] (0,-5) -- (0,5) node[left]{$y$} ;

  \draw [domain=-5:4,samples=200] plot (\x,-0.0012*\x^5-0.0131*\x*\x*\x*\x+0.0893*\x^3+0.3988*\x*\x-0.5881*\x-0.8857) ;
\end{tikzpicture}

EDIT: fixed it using the following post: How can I work around this TikZ bug: (\x)^2 and \x^2 produce different results in TikZ plot?. Using the following code fixes it completely.

\begin{tikzpicture}
  \draw [gray,dotted] (-6,-5) grid (5,5) ;
  \draw[->,>=latex] (-6,0) -- (5,0) node[below]{$x$} ;
  \draw[->,>=latex] (0,-5) -- (0,5) node[left]{$y$} ;

  \draw [domain=-5:4,samples=200] plot (\x,{-0.0012*\x^5-0.0131*(\x)^4+0.0893*\x^3+0.3988*(\x)^2-0.5881*\x-0.8857}) ;
\end{tikzpicture}