Problema con i mark in addplot

Intanto grazie per il benvenuto e per avermi risposto, mi scuso per non aver pensato fin da subito di riportare anche il preambolo con i pacchetti che uso. Quello riportato è quello completo che sto usando.
`
\documentclass[a4paper,11pt,twoside,openright,draft]{book}
\usepackage[italian]{babel}
\usepackage{graphicx}
%\graphicspath{{immag/}}
\usepackage{subfig}
\usepackage{indentfirst}
\usepackage{amsmath}
\usepackage[latin1]{inputenc}
\usepackage{booktabs}
\usepackage{multirow}
\usepackage{longtable}
\usepackage{lscape}
\usepackage{verbatim}
\usepackage{xkeyval}
\usepackage{siunitx}
\sisetup{output-decimal-marker={,}}
\usepackage{dcolumn}
\usepackage[margin=10pt,font=small,labelfont=bf,labelsep=endash,format=hang,justification=raggedright]{caption}
\captionsetup[table]{position=bottom}

\usepackage[table]{xcolor}
\usepackage{url}

\usepackage[pdfborder={0 0 0}]{hyperref}
\hypersetup{pdfborder=0 0 0}

\usepackage{pgfplots,relsize,pgfplotstable,lmodern,pgfkeys}
\usetikzlibrary{plotmarks}

\renewcommand{\thefigure}{\arabic{chapter}.\arabic{figure}}
\renewcommand{\thetable}{\arabic{chapter}.\arabic{table}}
\setcounter{tocdepth}{3}
\setcounter{secnumdepth}{3}

\DeclareMathOperator{\tg}{tg}
\DeclareMathOperator{\arctg}{arctg}
\newcommand{\grad}{$^\circ$}
\newcommand{\ud}{\mathrm{d}}

\makeatletter
\def\cleardoublepage{\clearpage\if@twoside \ifodd\c@page\else
\hbox{}
\thispagestyle{empty}
\newpage
\if@twocolumn\hbox{}\newpage\fi\fi\fi}
\makeatother

\pagestyle{plain}

\begin{document}
\chapter{Prova}

\begin{figure}
\centering

\pgfplotsset{
every axis x label/.append style = {
font = \relsize{0.9}
},
every axis y label/.append style = {
font = \relsize{0.9},
rotate = -90,
xshift = -0.5em
},
minor grid style={dotted}
}

\begin{tikzpicture}
\pgfplotsset{
width=8cm, height=15cm,
scale only axis,
no markers
}
%
%
\begin{axis} [
clip=false,
width=8cm,
hide y axis,
axis x line*=bottom,
xmin =0, xmax = 22,
ymin = -40, ymax = 0,
xtick={0,2,…,22},
minor xtick={0.2,0.4,…,21.8},
%ytick={0,-2,…,-40},
%minor ytick={-0.2,-0.4,…,-40},
xlabel = {$q_p$ (\si{\mega\pascal})},
]
\end{axis}
%
\begin{axis} [
clip=false,
height=15cm,
hide x axis,
axis y line*=left,
xmin =0, xmax = 22,
ymin = -40, ymax = 0,
ytick={0,-2,…,-40},
minor ytick={-0.2,-0.4,…,-40},
ylabel = \parbox{0.75cm}{\centering $y$ \\ \centering (\si{\meter})}
]
\end{axis}
%
%
\begin{axis}[
hide x axis,
hide y axis,
xmin=0, xmax=22,
ymin=-40, ymax=0
]
\tikzset{every mark/.append style={scale=0.5}}
\addplot[mark=o] coordinates{
(1,0)
(1.8,-0.2)
(1.6,-0.4)
(1.4,-0.6)
(0.8,-0.8)
(0.5,-1)
(0.7,-1.2)
(0.4,-1.4)
(1.3,-1.6)
(1.8,-1.8)
(3,-2)
(4.6,-2.2)
(5,-2.4)
(1.4,-2.6)
(0.5,-2.8)
(0.5,-3)
(2.6,-3.2)
(2.8,-3.4)
(0.7,-3.6)
(0.4,-3.8)
(0.3,-4)
(0.2,-4.2)
(0.6,-4.4)
(0.8,-4.6)
(2,-4.8)
(2.7,-5)
(2,-5.2)
(0.9,-5.4)
(1.6,-5.6)
};
\end{axis}
%
%
\begin{axis} [
clip=false,
width=8cm,
yshift=-1.5cm,
hide y axis,
axis x line*=bottom,
xmin =0, xmax = 0.5,
ymin = -40, ymax = 0,
xtick={0,0.1,…,0.5},
minor xtick={0.025,0.05,…,0.5},
xlabel = {$q_s$ (\si{\mega\pascal})},
]
\end{axis}
\begin{axis} [
hide x axis,
hide y axis,
xmin=0, xmax=0.5,
ymin=-40, ymax=0,
]
%\tikzset{every mark/.append style={scale=1}}
\addplot+ coordinates{
(0.01591,0)
(0.0282,-0.2)
(0.03059,-0.4)
(0.01781,-0.6)
(0.01548,-0.8)
(0.00704,-1)
(0.01393,-1.2)
(0.00587,-1.4)
(0.0238,-1.6)
(0.07299,-1.8)
(0.11651,-2)
(0.20558,-2.2)
(0.20811,-2.4)
(0.0589,-2.6)
(0.02092,-2.8)
(0.01825,-3)
(0.10927,-3.2)
(0.12132,-3.4)
(0.02564,-3.6)
(0.01713,-3.8)
(0.0133,-4)
(0.00709,-4.2)
(0.02641,-4.4)
(0.0334,-4.6)
(0.0758,-4.8)
};
\end{axis}
\end{tikzpicture}
\end{figure}

\end{document}
`

Per il problema del mark, introducevo \tikzset{every mark/.append style={scale=1}} solo per vedere se poteva esserci un effetto sulla dimensione, ma chiaramente che tipo di mark usare è specificato nelle dichiarazioni di addplot. La mia voleva essere solo una certezza, per non sentirmi rispondere che bastava aumentare il fattore di scala. Per il terzo punto ora andrò a leggere il manuale in merito alla legenda fuori dagli assi.