definecolor{mycolor1}{rgb}{0.00000,0.44700,0.74100}%
%
begin{tikzpicture}

begin{axis}[%
width=0.951fwidth,
height=fheight,
at={(0fwidth,0fheight)},
scale only axis,
xmin=0,
xmax=14,
ymin=-0.5,
ymax=0.5,
ylabel style={font=color{white!15!black}},
ylabel={Distance from centreline, $y$ [m]},
axis background/.style={fill=white},
xmajorgrids,
ymajorgrids
]
addplot [color=mycolor1, draw=none, mark=o, mark options={solid, mycolor1}, only marks, forget plot]
  table[row sep=crcr]{%
2.3 0\
12.7    -0.15\
};
label{addplot:line1}
addplot [color=black, draw=none, mark=asterisk, mark options={solid, black}, only marks, forget plot]
  table[row sep=crcr]{%
2.75    -0.3\
};
label{addplot:line2}
end{axis}
end{tikzpicture}%

begin{figure}
     setlengthfheight{0.55textwidth}
     setlengthfwidth{0.95textwidth}
     centering
     tikzsetnextfilename{tikzexternalrealjob-chapter1Figure1}%
      input{./ch1/img/Figure1.tikz}
          caption[Caption for list of figures without any references]{Caption with lines 1 (ref{addplot:line1}) and 2 (ref{addplot:line2}).}
          label{fig:drift_velocity_ak50}
end{figure}

! Package tikz Error: Sorry, the system call 'pdflatex -shell-escape-halt-on-error -interaction=batchmode -jobname "tikz/main-figure_crossref90" deftikzexternalrealjob{main}input{main}"' did NOT result in a usable output file 'tikz/main-figure_crossref90' (expected one of .pdf:.jpg:.jpeg:.png:). Please verify that you have enabled system calls. For pdflatex, this is 'pdflatex -shell-escape'. Sometimes it is also named 'write 18' or something like that. Or maybe the command simply failed? Error messages can be found in 'tikz/main-figure_crossref90.log'. If you continue now, I'll try to typeset the picture.

pdflatex -shell-escape -synctex=1 -interaction=nonstopmode main.tex