[Tex/LaTex] Draw Arrows outside of content-area of TikZ

pgfplotstikz-pgf

I have a rather standard tikz plot. To clarify that low values on the y axis are actually faster/better I want to draw an additional arrow outside of the actually plotting area as shown in the attachment.
I found various ways of drawing using \draw but they all operate only within the content area of the plot – how can I draw outside of this area?

Here is a MWE of how I create my plot at the moment:

\documentclass[11pt,a4paper]{article}
\usepackage{pgfplots, pgfplotstable}    
\usepackage{tikz}

\begin{document}
\begin{figure}
\begin{tikzpicture}[scale=0.75]
         \pgfplotsset{grid style={dashed,gray}}
        \begin{axis}[
        xlabel={X}, 
        ylabel=time,
        xmin=0.8,
        xmax=1.0,
        ymin=1,     
        ymax=200,   
        xmajorgrids=true,
        ymajorgrids=true,
    ]      

\addplot+[
            black,
            mark options={fill= black},
            only marks,
            mark size=2,
            mark=square*,
        ] 
        coordinates {
            (0.805, 10)
            (0.85, 20)

        };



    \end{axis}
    \end{tikzpicture}   

\end{figure}

\end{document}

enter image description here

Best Answer

We name the axis, so as to be able to access it outside the axis environment.

Then, we draw a line between some key points of it.

The output

enter image description here

The code

\documentclass[tikz]{standalone}
\usepackage{pgfplots}
\usetikzlibrary{arrows}

\begin{document}
\begin{tikzpicture}[scale=0.75, >=stealth']
  \pgfplotsset{grid style={dashed,gray}}
  \begin{axis}
    [
      name=myGraph,
      xlabel={X}, 
      ylabel=time,
      xmin=0.8,
      xmax=1.0,
      ymin=1,     
      ymax=200,   
      xmajorgrids=true,
      ymajorgrids=true,
    ]      

    \addplot+
    [
      black,
      mark options={fill= black},
      only marks,
      mark size=2,
      mark=square*,
    ] 
    coordinates 
    {
      (0.805, 10)
      (0.85, 20)
    };
  \end{axis}

  \def\myShift{-2cm}
  \draw [red, very thick, ->] ([xshift=\myShift]myGraph.north west) -- ([xshift=\myShift]myGraph.south west) node [midway, rotate=90, fill=white, yshift=2pt] {faster} ;
  %\draw [red, very thick, ->] (myGraph.left of north west) -- (myGraph.left of south west) node [midway, rotate=90, fill=white, yshift=2pt] {faster} ; % an alternative way
\end{tikzpicture}
\end{document}

Cheers,

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