Reference of plot not resolved

Question

The label of the first plot (added by \addlegendimage) appears in default black after two compilations which indicates that the reference of the first plot is not being resolved. Here is the code:

\documentclass[12pt,twoside,openright]{report}
\usepackage{tikz,pgfplots,float}
\usepackage{csvsimple}
\usepgfplotslibrary{external}
\usetikzlibrary{external}
\tikzexternalize
\pgfplotsset{compat=newest}
\begin{document}
\begin{filecontents}{curvas_tension_DC_y_vel_desacoplamiento_OWB_Hor.csv}
t;v_DC;vel_DCPM;vel_AFPM
0;0;0;0
0.5;0.5;1.25;0.125
1;1;5;1
1.5;1.5;11.25;3.375
2;2;20;8
2.5;2.5;31.25;15.625
3;3;45;27
3.5;3.5;61.25;42.875
4;4;80;64
4.5;4.5;101.25;91.125
5;5;125;125
5.5;5.5;151.25;166.375
6;6;180;216
6.5;6.5;211.25;274.625
\end{filecontents}
\begin{figure}[!ht]
\centering
\begin{tikzpicture}
\begin{axis}[
ymin=0, ymax=6.75,
xmin=0, xmax=6.75,
grid,
xtick={0,0.5,1,1.5,2,2.5,3,3.5,4,4.5,5,5.5,6,6.5},
ytick={0,0.5,1,1.5,2,2.5,3,3.5,4,4.5,5,5.5,6,6.5},
axis y line=left, % the '' avoids arrow heads
xlabel={$t$ [s]},
xlabel near ticks,
ylabel={$v_{fuente}$ [V]},
ylabel near ticks,
width=\textwidth, 
height=0.7\textwidth,
]
\addplot[ultra thick,violet!17] table[x=t, y=v_DC,col sep=semicolon]{curvas_tension_DC_y_vel_desacoplamiento_OWB_Hor.csv}; \label{plot_one}
\end{axis}
\begin{axis}[
ymin=0, ymax=325,
xmin=0, xmax=6.75,
ytick={0,50,100,150,200,250,300},
hide x axis,
axis y line*=right,
ylabel={Velocidad [rpm]},
ylabel near ticks,
width=\textwidth, 
height=0.7\textwidth,
]
\addlegendimage{/pgfplots/refstyle=plot_one}\addlegendentry{$v_{fuente}$}
\addplot[no markers,teal] table[x=t, y=vel_DCPM,col sep=semicolon]{curvas_tension_DC_y_vel_desacoplamiento_OWB_Hor.csv};
\addlegendentry{$n _{m , in}$};
\addplot[no markers,orange] table[x=t, y=vel_AFPM,col sep=semicolon]{curvas_tension_DC_y_vel_desacoplamiento_OWB_Hor.csv};
\addlegendentry{$n _{m , out}$};
\end{axis}
\end{tikzpicture}
\end{figure}
\end{document}

Here you can find the .csv file.

For this I followed the example in the accepted answer in here.

Thanks in advance.