Pgfplots and Groupplot: Legend Management

Question

I have the following MWE:

\documentclass{article}
\usepackage[dvipsnames]{xcolor}
\usepackage{pgf,pgfplots,pgfplotstable}
\pgfplotsset{compat=newest}
\usepgfplotslibrary{colormaps}
\usepgfplotslibrary{groupplots}
\begin{filecontents}{data1.dat}
   t1       a1              t2       a2               t3        a3 
0.142    1.000           0.154    1.000            0.139     0.995
0.284    1.000           0.308    1.000            0.279     0.990
0.426    1.000           0.461    1.000            0.418     0.989
0.569    1.000           0.615    1.000            0.558     0.989
0.711    1.000           0.769    1.000            0.697     0.989
0.853    1.000           0.923    1.000            0.836     0.989
0.995    1.000           1.077    1.000            0.976     0.989
1.137    1.000           1.230    1.000            1.115     0.989
1.279    1.000           1.384    1.000            1.255     0.989
1.422    0.995           1.538    1.000            1.394     0.986
1.564    0.990           1.692    1.000            1.534     0.981

\end{filecontents}
\begin{filecontents}{data2.dat}
   t1       a1              t4       a4               t3        a3
3.554    0.920           3.845    0.950            3.485     0.913
3.696    0.915           3.999    0.945            3.625     0.908
3.838    0.910           4.153    0.939            3.764     0.903
3.980    0.905           4.306    0.934            3.904     0.898
4.123    0.900           4.460    0.929            4.043     0.893
4.265    0.895           4.614    0.923            4.182     0.888
4.407    0.890           4.768    0.918            4.322     0.883
4.549    0.885           4.922    0.912            4.461     0.878
4.691    0.880           5.075    0.907            4.601     0.873
4.833    0.875           5.229    0.902            4.740     0.868
4.976    0.870           5.383    0.896            4.880     0.864
5.118    0.865           5.537    0.891            5.019     0.859
5.260    0.860           5.691    0.885            5.158     0.854

\end{filecontents}
\begin{document}
\begin{figure}[htbp]
\centering
\begin{tikzpicture}[scale=0.9]
\begin{groupplot}[group style={
    group name=rzrxcomp,
    group size=1 by 2,
    xlabels at=edge bottom,
    ylabels at=edge left,
    vertical sep=4em
    },
    xmin = 0, xmax = 2,
    ymin = 0.8, ymax = 1,
    xtick distance = 0.2,
    ytick distance = 0.2,
    xlabel={$time;[s]$},
    ylabel={$mass$},
    grid = both,
    grid style = {dotted},
    minor tick num = 1,
    major grid style = {lightgray!75},
    minor grid style = {lightgray!75},
    width = 0.85\textwidth,
    height = 0.50\textwidth,
    %scale only axis,
    %legend cell align = {left},
    %legend pos = north east
    legend style={at={(0.5,-0.3)},
    anchor=north,legend columns=-1},
    legend cell align = {left},
]
\nextgroupplot
\addplot[smooth, black, very thick] table [x = {t1}, y = {a1}] {data1.dat};
\addplot[smooth, Lavender, very thick] table [x = {t2}, y = {a2}] {data1.dat};
\addplot[smooth, Violet, very thick] table [x = {t3}, y = {a3}] {data1.dat};
\nextgroupplot[legend entries={n1,n2,n3,n4}]
\addlegendimage{black, very thick, no markers}
\addlegendimage{Lavender, very thick, no markers}
\addlegendimage{Violet, very thick, no markers}
\addlegendimage{BurntOrange, dotted, very thick, no markers}
\addplot[smooth, black, very thick] table [x = {t1}, y = {a1}] {data2.dat};
\addplot[smooth, BurntOrange, dotted, very thick] table [x = {t4}, y = {a4}] {data2.dat};
\addplot[smooth, Violet, very thick] table [x = {t3}, y = {a3}] {data2.dat};
%\legend{
%n1,
%n2,
%n3,  
%}


\end{groupplot}
\end{tikzpicture}
\end{figure}
\end{document}

Now imagine that we consider: (t1,a1)=n1, (t2,a2)=n2, (t3,a3)=n3, (t4,a4)=n4. Is there a way to add the second column-set of values in data2.dat, i.e. (t4, a4), as n4 inside the legend? The issue is that I don't know how to add inside the legend n4 without messing up with n2, which is in the same position of n4 but in the first \groupplot. I need to keep one single legend, highlighting the four different plots with the related different colors.

UPDATE

Following @TorbjornT suggestion, I've used the \addlegendimage command in order to deal with plots' order and names in one single legend, and it works fine. The MWE has been updated with the new part. Now I have another question, how can I make solid tick, very tick... lines inside the generated legend? The \addlegendimage command doesn't seem to recognize or accept those style features.