pgfplot will not plot past a x value, plot is cut off

Question

I tried plotting the following functions; however, they are cut off at x=[-5,5] and I cannot figure out why. Any help would greatly be appreciated.

\documentclass{article}
\usepackage{pgfplots}

\usepgfplotslibrary{external}
\tikzexternalize[shell escape=-enable-write18] %needed for the MiKTeX compiler
\tikzset{external/force remake}

\begin{document}
    \begin{tikzpicture}
        \begin{axis}[xmin=-10,xmax=10]
            \addplot [mark=none, smooth, color=blue]{x^2+2};
            \addplot [mark=none, smooth, color=red]{x^2};
            \addplot [mark=none, smooth, color=green]{x^2-2};
            \addlegendentry{$x^2+2$}
            \addlegendentry{$x^2$}
            \addlegendentry{$x^2-2$}
        \end{axis}
    \end{tikzpicture}

    \begin{tikzpicture}
            \begin{axis}[xmin=-10,xmax=10]
                \addplot [mark=none, smooth, color=blue]{(x+2)^2};
                \addplot [mark=none, smooth, color=red]{x^2};
                \addplot [mark=none, smooth, color=green]{(x-2)^2};
                \addlegendentry{$(x+2)^2$}
                \addlegendentry{$x^2$}
                \addlegendentry{$(x-2)^2$}
            \end{axis}
    \end{tikzpicture}
\end{document}

Answer 1

You need to tell each \addplot the domain to plot over, i.e.

\addplot [mark=none, smooth, color=blue, domain=-10:10]{x^2+2};

As noted by Knabber, a short-cut is to place the domain boundaries in the axis options, i.e.

\begin{axis}[xmin=-10,xmax=10, domain=-10:10]
  \addplot [mark=none, smooth, color=blue]{x^2+2};
  \addplot [mark=none, smooth, color=red]{x^2};
  \addplot [mark=none, smooth, color=green]{x^2-2};
  ...
\end{axis}

Finally, if you don't like repetition you can do

 \begin{axis}[enlargelimits=false, domain=-10:10]

where the enlargelimits=false makes the axis boundary hug the domain (no padding).