Tikzpicture - Arrow next to/under plot label

Question

I am looking for a help with Latex. I'm new here so unfortunately I'm not allowed to post pictures yet.

I have a tikzpicture plot with labeled axes (x and y). Now I would like to add an arrow below the x-axis to indicate the direction of the subsequential treatments. In addition I would like to have an arrow left of the y-axis label pointing upwards. This arrow should have a second label saying "relative increase in Fe".

\begin{figure}[htbp]
\centering
\begin{tikzpicture}[scale=1]
\begin{axis}[
    %legend style={at={(1.05,0.05)}, %gibt Ort für Legende an
    %anchor=south west},
    %axis x line=bottom,    % erzeugt x-Achse mit Pfeil
    %axis y line=left,  %
    width=15.5cm,
    height=10cm,
    %scaled ticks=false,
    %ymin=0,
    xmin=-0.5,
    xmax=5,
    ymin=0,
    ymax=5,
    xtick={0,1,2,3,4},
    xticklabels={Fe2O3,1,2,3,4},
    bar width=50pt,
    %ytick={},
    %yticklabels={},
                %use un%%ts,
                %x unit=-,
                %x unit prefix=,
                %y unit=\frac{m}{s},
                %y unit prefix=,
        xlabel=Subsequential Treatments over Time ,
    ylabel=Auger Peak to Peak Height Ratio Fe:O,
            x tick label style= {rotate=90,anchor=east},
            ybar stacked]



    \addplot [draw=white, very thin]
            coordinates {(0,1) (1,1) (2,3) (3,2) (4,1.5)}; 
            \addplot [draw= blue, fill=blue]
            coordinates {(0,1) (1,1) (2,3) (3,2) (4,1.5)}; 


            %\node at (100,1) [orange!50!yellow]{\small{ZnO-h}};


\end{axis}
\end{tikzpicture}

\caption[Auger Spectrum of HOPG]{Auger Peak to Peak Height Ratios of Fe:O at an primary electron beam   of \unit{2.0}{keV}.}
 \label{Auger_ratio_histogram_}
\end{figure}

Answer 1

The code below illustrates two techniques for placing such arrows. The starting point of both is to add name=MyAxis to the axis options, which allows you to refer to the anchor points of the axis as with a normal node. A pgfplots axis also has anchors such as outer north east, which lies outside the axis descriptions, while north east lies on the corner of the axes.

\documentclass[border=5mm]{standalone}
\usepackage{pgfplots}
\begin{document}
\begin{tikzpicture}
\begin{axis}[
    name=MyAxis,
    %legend style={at={(1.05,0.05)}, %gibt Ort für Legende an
    %anchor=south west},
    %axis x line=bottom,    % erzeugt x-Achse mit Pfeil
    %axis y line=left,  %
    width=15.5cm,
    height=10cm,
    %scaled ticks=false,
    %ymin=0,
    xmin=-0.5,
    xmax=5,
    ymin=0,
    ymax=5,
    xtick={0,1,2,3,4},
    xticklabels={Fe2O3,1,2,3,4},
    bar width=50pt,
    %ytick={},
    %yticklabels={},
                %use un%%ts,
                %x unit=-,
                %x unit prefix=,
                %y unit=\frac{m}{s},
                %y unit prefix=,
        xlabel=Subsequential Treatments over Time ,
    ylabel=Auger Peak to Peak Height Ratio Fe:O,
            x tick label style= {rotate=90,anchor=east},
            ybar stacked]



    \addplot [draw=white, very thin]
            coordinates {(0,1) (1,1) (2,3) (3,2) (4,1.5)}; 
            \addplot [draw= blue, fill=blue]
            coordinates {(0,1) (1,1) (2,3) (3,2) (4,1.5)}; 


            %\node at (100,1) [orange!50!yellow]{\small{ZnO-h}};


\end{axis}
\draw [-latex] ([yshift=-2ex]MyAxis.outer south west) --node[below]{Direction of subsequential treatments} ([yshift=-2ex]MyAxis.outer south east);
\draw [-latex] (MyAxis.outer south west) ++(-2ex,0) coordinate(start) --node[sloped,above] {relative increase in Fe} (start |- MyAxis.outer north west);
\end{tikzpicture}
\end{document}