Save tikz image and use away from latex

Question

I need to create and save latex drawing so i can use there else where.

Im using the code

\begin{tikzpicture}[scale=0.8,domain=-2:20]
\begin{axis}[
height = 11.7cm,
width = 15cm,
  axis lines=middle,
  grid=major,
  xmin=-2,
  xmax=10,
  ymin=-2,
  ymax=20,
   y label style={at={(axis description cs:0.5,-0.05)},anchor=north},
    x label style={at={(axis description cs:-0.05,.5)},rotate=0,anchor=south},
  xlabel=$y$,
  ylabel=$x$,
  xtick={-2,-1,0,...,14},
  ytick={-2,0,...,20},
  tick style={very thick},
  legend style={
  at={(rel axis cs:0,1)},
  anchor=north west,draw=none,inner sep=0pt,fill=gray!10}
]
\addplot[black,thick,samples=1] {0};
\draw[color=red, smooth] plot (\x,\x*\x-8*\x+16) node[above left] {$y = 3$};
\draw (4,1.5) node[anchor=north]{(4,0)};
\draw (1.2,16) node[anchor=east]{(0,16)};
\draw (7,8)  node[anchor=west]{$y=x^2-8x+16$};
\end{axis}
\end{tikzpicture}  

and i want to save the image it creates as a png. I need to create multiple of these with differences to each image. So i need to create all the images with the exact scale and size so doing a screenshot of the page is not good enough.

From the comments I have developed the code including the preamble from my original document.

\documentclass[tikz,convert]{standalone} 
\usepackage[colorlinks=true]{hyperref}
\usepackage[english]{babel}
\usepackage[utf8x]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{amssymb}
\usepackage{amsmath}
\usepackage{amsthm}
\usepackage{amsfonts}
\usepackage{graphicx}
\usepackage[subpreambles=true]{standalone}
\usepackage{import}
\usepackage{mathtools}
\usepackage{tkz-euclide}
\usepackage{bigints}
\usetkzobj{all}
\usepackage{verbatim}
\usepackage{array}
\usepackage{float}
\usepackage{tikz}
\usepackage{pgfplots}
\usepackage{siunitx}
\usepackage{gensymb}


\usepackage{xlop}

\usepackage{color,soul}
\usepackage[colorinlistoftodos]{todonotes}
\usepackage[a4paper,top=3cm,bottom=2cm,left=3cm,right=3cm,marginparwidth=1.5cm]{geometry}
\usetikzlibrary{intersections}
\sisetup{output-decimal-marker={,}}
\pgfplotsset{compat=1.14}
\setlength\parindent{0pt}
\setlength{\parskip}{2em}
\renewcommand{\arraystretch}{1.9}

%need for table solutions
\usepackage{xcolor}
\CorrectChoiceEmphasis{\color{red}}
\setlength\fillinlinelength{1cm}
\setlength\answerclearance{0.5ex}

\newcommand{\NN}{\mathbb{N}}
\newcommand{\ZZ}{\mathbb{Z}}
\newcommand{\CC}{\mathbb{C}}
\newcommand{\RR}{\mathbb{R}}
\newcommand*\circled[1]{\tikz[baseline=(char.base)]{\node[shape=circle,draw,inner sep=2pt] (char) {#1};}}
\newcommand{\sinth}{\sin(\theta)}
\newcommand{\costh}{\cos(\theta)}
\newcommand{\tanth}{\tan(\theta)}
\newcommand{\secth}{\sec(\theta)}
\newcommand{\cosecth}{\cosec(\theta)}
\newcommand{\coshth}{\cosh(\theta)}
\newcommand{\sinhth}{\sinh(\theta)}
\newcommand{\tanhth}{\tanh(\theta)}

\DeclareMathOperator{\sininv}{sin^{-1}}
\DeclareMathOperator{\cosinv}{cos^{-1}}
\DeclareMathOperator{\taninv}{tan^{-1}}

\theoremstyle{definition}
\newtheorem{exercise}{Exercise}[section]
\theoremstyle{definition}
\newtheorem{example}{Example}[section]





\begin{document}
\begin{tikzpicture}[domain=-2:20, range=-2:10]
\begin{axis}[
  axis lines=middle,
  grid=major,
  xmin=-2,
  xmax=10,
  ymin=-2,
  ymax=20,
   y label style={at={(axis description cs:0.5,-0.05)},anchor=north},
    x label style={at={(axis description cs:-0.05,.5)},rotate=0,anchor=south},
  xlabel=$y$,
  ylabel=$x$,
  xtick={-2,-1,0,...,14},
  ytick={-2,0,...,20},
  tick style={very thick},
  legend style={
  at={(rel axis cs:0,1)},
  anchor=north west,draw=none,inner sep=0pt,fill=gray!10}
]
\addplot[black,thick,samples=1] {0};
\draw[color=red, smooth] plot (\x,\x*\x-8*\x+16) node[above left] {$y = 3$};
\draw (4,1.5) node[anchor=north]{(4,0)};
\draw (1.2,16) node[anchor=east]{(0,16)};
\draw (7,8)  node[anchor=west]{$y=x^2-8x+16$};
\end{axis}
\end{tikzpicture}  
\end{document}

This creates my graph but not in the centre of the page and the graph goes over the page.

Using the code given below for the equation when it works perfectly. However if i try and do another graph it goes over the page.

Answer 1

I'll be happy to remove this but if I compile

\documentclass[tikz,convert]{standalone} 
\usepackage{pgfplots} 
\pgfplotsset{compat=1.16} 
\begin{document}
\begin{tikzpicture}[scale=0.8,domain=-2:20]
\begin{axis}[
height = 11.7cm,
width = 15cm,
  axis lines=middle,
  grid=major,
  xmin=-2,
  xmax=10,
  ymin=-2,
  ymax=20,
   y label style={at={(axis description cs:0.5,-0.05)},anchor=north},
    x label style={at={(axis description cs:-0.05,.5)},rotate=0,anchor=south},
  xlabel=$y$,
  ylabel=$x$,
  xtick={-2,-1,0,...,14},
  ytick={-2,0,...,20},
  tick style={very thick},
  legend style={
  at={(rel axis cs:0,1)},
  anchor=north west,draw=none,inner sep=0pt,fill=gray!10}
]
\addplot[black,thick,samples=1] {0};
\draw[color=red, smooth] plot (\x,\x*\x-8*\x+16) node[above left] {$y = 3$};
\draw (4,1.5) node[anchor=north]{(4,0)};
\draw (1.2,16) node[anchor=east]{(0,16)};
\draw (7,8)  node[anchor=west]{$y=x^2-8x+16$};
\end{axis}
\end{tikzpicture}  
\end{document}

with pdflatex -shell-escape I get a png file of the right size.

Thanks for adding a preamble. Unfortunately, your code throws errors and, related to it, the preamble is by far not minimal. After stripping off all things that are not related to the tikzpicture one arrives at

\documentclass[tikz,convert]{standalone} 
\usepackage{pgfplots}
\pgfplotsset{compat=1.14}
\begin{document}
\begin{tikzpicture}[domain=-2:20, range=-2:10]
\begin{axis}[
  axis lines=middle,
  grid=major,
  xmin=-2,
  xmax=10,
  ymin=-2,
  ymax=20,
   y label style={at={(axis description cs:0.5,-0.05)},anchor=north},
    x label style={at={(axis description cs:-0.05,.5)},rotate=0,anchor=south},
  xlabel=$y$,
  ylabel=$x$,
  xtick={-2,-1,0,...,14},
  ytick={-2,0,...,20},
  tick style={very thick},
  legend style={
  at={(rel axis cs:0,1)},
  anchor=north west,draw=none,inner sep=0pt,fill=gray!10}
]
\addplot[black,thick,samples=1] {0};
\draw[color=red, smooth] plot (\x,\x*\x-8*\x+16) node[above left] {$y = 3$};
\draw (4,1.5) node[anchor=north]{(4,0)};
\draw (1.2,16) node[anchor=east]{(0,16)};
\draw (7,8)  node[anchor=west]{$y=x^2-8x+16$};
\end{axis}
\end{tikzpicture}  
\end{document}

which produces, when compiled with pdflatex -shell-escape this very png file:

EDIT: Type fixed, pgflatex -> pdflatex, thanks to quark67!