كيف أضع المفتاح أسفل المبيان على برنامج LaTeX؟

الخطأ كان في تحديد مكان المفتاح، حيث أنه يظهر افتراضيا بجانب المبيان، لذا كان من الواجب إضافة سطر فارغ ليتمكن من الظهور في الأسفل.

الكود بعد التصحيح:

\documentclass[border=10pt]{standalone}
\usepackage{pgfplots}

\begin{document}
    \begin{tikzpicture}

    \begin{axis}[ybar stacked,
    legend style={                    % this is modified
        legend cell align=left,       %
        legend columns=2,             %
        at={(xticklabel cs:0.5)},     %
        anchor=north                  % up to here
    }, area legend,
    x tick label style={ /pgf/number format/1000 sep=},
    ylabel=Revenue (\$\,billions),
    enlargelimits=0.05,width=10cm,height=12cm,
    bar width=12pt,
    ]
    \addplot coordinates
    {(2012,1388) (2013,1388) (2014,1388) (2015,1388) (2016,1388) (2017,1388) (2018,1500)};
    \addlegendentry{\textsc{InkJet Head}} 

    \addplot coordinates
    {(  2012    ,   2995    ) ( 2013    ,   3144.75 ) ( 2014    ,   3294.5  ) ( 2015    ,   2126.45 ) ( 2016    ,   3594    ) ( 2017    ,   3893.5  ) ( 2018    ,   4342.75 ) }; 
    \addlegendentry{\textsc{Pressure sensors}} 

    \addplot  coordinates
    {(  2012    ,   3594    ) ( 2013    ,   3923.45 ) ( 2014    ,   4193    ) ( 2015    ,   4432.6  ) ( 2016    ,   4792    ) ( 2017    ,   5091.5  ) ( 2018    ,   5450.9  ) };
    \addlegendentry{\textsc{Microphones}} 

    \addplot [color=yellow, fill=yellow] coordinates
    {(  2012    ,   5361.05 ) ( 2013    ,   5450.9  ) ( 2014    ,   5690.5  ) ( 2015    ,   5960.05 ) ( 2016    ,   6289.5  ) ( 2017    ,   6529.1  ) ( 2018    ,   6738.75 )};     
    \addlegendentry{\textsc{Accelerometers}} 

    \addplot [color=black, fill=white] coordinates
    {(  2012    ,   6648.9  ) ( 2013    ,   6888.5  )  (    2014    ,   7128.1  ) ( 2015    ,   7188    ) ( 2016    ,   7457.55 ) ( 2017    ,   7637.25 ) ( 2018    ,   7787    )};     
    \addlegendentry{\textsc{Gyroscopes}} 

    \addplot [color=violet, fill=violet] coordinates
    {(  2012    ,   6948.4  ) ( 2013    ,   7337.75 )  (    2014    ,   7547.4  ) ( 2015    ,   7727.1  ) ( 2016    ,   7787    )  (    2017    ,   8086.5  ) ( 2018    ,   8236.25 )};     
    \addlegendentry{\textsc{Digital compass}} 

    \addplot [color=gray, fill=gray] coordinates
    {(  2012    ,   7337.75 ) ( 2013    ,   7906.8  ) ( 2014    ,   8445.9  )  (    2015    ,   9164.7  ) ( 2016    ,   9584) ( 2017    ,   10033.25    ) ( 2018    ,   10482.5 ) };        
    \addlegendentry{\textsc{Inertial combos}} 

    \addplot [color=purple, fill=purple] coordinates
    {(2012,7487.5) (2013,8176.35) (2014,8835.25) (2015,9494.15) (2016,10033.25) (2017,10632.25) (2018,11021.6)};        
    \addlegendentry{\textsc{Microbolometers}} 


    \addplot [color=pink, fill=pink] coordinates
    {(  2012    ,   7547.4  ) ( 2013    ,   8236.25 ) ( 2014    ,   8850.225    ) ( 2015    ,   9509.125    ) ( 2016    ,   10048.225   ) ( 2017    ,   10647.225   ) ( 2018    ,   11081.5 ) };
    \addlegendentry{\textsc{PIR \& thermopiles}} 


    \addplot [color=olive, fill=olive] coordinates
    {(  2012    ,   7562.375    ) ( 2013    ,   8251.225    ) ( 2014    ,   8865.2  ) ( 2015    ,   9524.1  ) ( 2016    ,   10063.2 ) ( 2017    ,   10662.2 ) ( 2018    ,   11096.475   ) };
    \addlegendentry{\textsc{Micro displays}} 


    \addplot [color=magenta, fill=magenta] coordinates
    {(  2012    ,   8386    ) ( 2013    ,   9134.75 ) ( 2014    ,   10033.25    ) ( 2015    ,   10886.825   ) ( 2016    ,   11740.4 ) ( 2017    ,   12608.95    ) ( 2018    ,   13477.5 ) };
    \addlegendentry{\textsc{Projection systems}}


    \addplot [color=lime, fill=lime] coordinates
    {(  2012    ,   8685.5  ) ( 2013    ,   9434.25 ) ( 2014    ,   10332.75    )  (    2015    ,   11081.5 ) ( 2016    ,   12129.75    ) ( 2017    ,   12938.4 ) ( 2018    ,   13926.75    ) };
    \addlegendentry{\textsc{Other optical MEMS}}


    \addplot [color=green, fill=green] coordinates
    {(  2012    ,   8985    ) ( 2013    ,   10003.3 ) ( 2014    ,   10931.75    ) ( 2015    ,   12219.6 ) ( 2016    ,   13417.6 ) ( 2017    ,   14825.25    ) ( 2018    ,   16472.5 ) };
    \addlegendentry{\textsc{Microfluidics for research}} 


    \addplot [color=cyan, fill=cyan] coordinates
    {(  2012    ,   9823.6  ) ( 2013    ,   10931.75    ) ( 2014    ,   12129.75    ) ( 2015    ,   13537.4 ) ( 2016    ,   15124.75    ) ( 2017    ,   16831.9 ) ( 2018    ,   19018.25    ) };
    \addlegendentry{\textsc{Microfluidics for IVD}}


    \addplot [color=brown, fill=brown] coordinates
    {(  2012    ,   10123.1 ) ( 2013    ,   11231.25    ) ( 2014    ,   12579   ) ( 2015    ,   14076.5 ) ( 2016    ,   15723.75    )  (    2017    ,   17520.75    ) ( 2018    ,   18419.25    ) };
    \addlegendentry{\textsc{Microdispensers}}


    \addplot [color=red, fill=red] coordinates
    {(  2012    ,   10482.5 ) ( 2013    ,   11830.25    ) ( 2014    ,   13327.75    ) ( 2015    ,   14825.25    ) ( 2016    ,   16622.25    ) ( 2017    ,   18628.9 ) ( 2018    ,   21114.75    ) };
    \addlegendentry{\textsc{RF MEMS}}


    \addplot [color=orange, fill=orange] coordinates
    {(  2012    ,   10542.4 ) ( 2013    ,   11860.2 ) ( 2014    ,   13447.55    ) ( 2015    ,   14975   ) ( 2016    ,   16921.75    ) ( 2017    ,   19018.25    ) ( 2018    ,   21564   ) };
    \addlegendentry{\textsc{Oscillators}}   


    \addplot [color=black, fill=black] coordinates
    {(  2012    ,   10871.85    )  (    2013    ,   12279.5 ) ( 2014    ,   13777   ) ( 2015    ,   17071.5 ) ( 2016    ,   17371   ) ( 2017    ,   19617.25    ) ( 2018    ,   22312.75    ) };
    \addlegendentry{\textsc{Others}}

    %\legend{InkJet Head, Pressure sensors, Microphones,  Accelerometers, Gyroscopes, Digital compass, Inertial combos, Microbolometers, PIR \& thermopiles, Micro displays, Projection systems, Other optical MEMS, Microfluidics for research, Microfluidics for IVD, Microdispensers, RF MEMS, Oscillators, Others}
    \end{axis}

    \end{tikzpicture}
\end{document}

النتيجة النهائية: