Dual axis Plot problem

Question

\documentclass{article}
\usepackage{pgfplots, tikz}
\usepackage{adjustbox}  % table scale


\begin{document}
\begin{figure}
\centering
\begin{adjustbox}{max width=.75\textwidth}
\begin{tikzpicture}
    \pgfplotsset{
        symbolic x coords={40,60,80,100,120},
        xtick=data,
        legend columns=-1,
        legend style={draw=none},
        legend to name=named,
    }

    \begin{axis}[
    axis y line*=left,
    xlabel=x-axis,
    ylabel=y-axis 1,
    ybar stacked, ymin=0,
    bar width=7mm, 
    legend entries={a,b},
    ]
    \addplot [fill=blue] coordinates {
        ({40},15)
        ({60},25)
        ({80},35)
        ({100},15)
        ({120},10)
    };
    \addplot [fill=red] coordinates {
        ({40},10)
        ({60},35)
        ({80},30)
        ({100},25)
        ({120},10)
    };
    \end{axis}

    \begin{axis}[
    axis y line*=right,
    ylabel=y-axis 2, legend entries={time},
    ]


    \addplot[smooth,mark=*,black]
    coordinates{
        ({40},15)
        ({60},25)
        ({80},35)
        ({100},15)
        ({120},10)
    };
    \end{axis}

    \end{tikzpicture}
\end{adjustbox}
\\
Sample: \ref{named}
\end{figure}
\end{document}

Problems:

1. y-axis 2 label not appearing properly (appears left).

2. Legends not showing for stacked plots.

3. The text "Sample" and the legend seems to not appearing horizontally in the same line.

Answer 1

This solution is a combination of @cfr answer here and @soapygopher answer here. The double text could be removed, by adding only one axis label. The legend position is defined with legend style={at={(0.5,-0.2)},anchor=north} for both axis enviroments.

Here is the updated answer, because of question 3. You could add some text with inserting an extra column legend columns=4 instead of legend columns=3 with legend style you could do some format changes like text width. To add the extra text/titel use \addlegendimage{empty legend} and \addlegendentry{\textbf{Sample:}}. To add the second ylabel on the right side use ylabel=y-axis 2, ylabel near ticks, yticklabel pos=right, instead of axis y line*=right,:

\documentclass{article}
\usepackage{pgfplots, tikz}
\usepackage{adjustbox}  % table scale


\begin{document}

\begin{tikzpicture}
    \pgfplotsset{
        symbolic x coords={40,60,80,100,120},
        xtick=data,
        legend columns=4,
        legend style={
                    /tikz/every even column/.append style={text width=1.4cm}
                        },
    }

    \begin{axis}[
    axis y line*=left,
    xlabel=x-axis 1,
    ylabel=y-axis 1,
    ybar stacked, ymin=0,
    bar width=7mm, legend style={at={(0.5,-0.2)},anchor=north}
    ]

    \addplot [fill=blue,draw=none,area legend] coordinates {
        ({40},15)
        ({60},25)
        ({80},35)
        ({100},15)
        ({120},10)
    };\label{plot_one}
\addlegendentry{plot 1}
    \addplot [fill=red,draw=none,area legend] coordinates {
        ({40},10)
        ({60},35)
        ({80},30)
        ({100},25)
        ({120},10)
    };\label{plot_two}
\addlegendentry{plot 2}
    \end{axis}

 \begin{axis}[
    ylabel=y-axis 2, ylabel near ticks, yticklabel pos=right,legend style={at={(0.5,-0.2)},anchor=north},
    ]
\addlegendimage{empty legend}
\addlegendentry{\hspace*{0cm}\textbf{Sample:}}
\addlegendimage{/pgfplots/refstyle=plot_one}\addlegendentry{plot 1}
\addlegendimage{/pgfplots/refstyle=plot_two}\addlegendentry{plot 2}
    \addplot[smooth,mark=*,black]
    coordinates{
        ({40},15)
        ({60},25)
        ({80},35)
        ({100},15)
        ({120},10)
    };
    \addlegendentry{plot 3}
    \end{axis}

    \end{tikzpicture}

\end{document}

Or with 2x \begin{axis}[...,ymin=0,ymax=70, ...] you get: