How to make graph with both negative values in tikzpicture

Question

I have found a small issue with negative values graph in tikzpicture. My source code is very simple:

\documentclass[12pt, a4paper]{report}

\usepackage[czech]{babel}
\usepackage[utf8]{inputenc}
\usepackage{tikz} 
\usepackage{pgfplots}

\begin{document}
\begin{figure}[ht!]
\center
\begin{tikzpicture}[]
    \begin{axis}[xlabel={Power (dBm)}, ylabel={log BER (---)},
                                width=10cm,
                                height=8cm,
                                xmin=-10,   xmax=-25,
                                ymin=-1,  ymax=-25, 
                                legend pos=north east, grid=major]%or north west, ... ,outer north east
                \addplot file[skip first] {graphs/1-32.txt};
                \addplot file[skip first] {graphs/1-64.txt};
        \addlegendentry{1:32}        
                \addlegendentry{1:64}
   \end{axis}
\end{tikzpicture}
\caption{Power vs. log BER}
\label{}
\end{figure}

\end{document}

I can include my 1:32.txt file:

x  y  
-18.28  -20.00392635
-19.42  -16.09799711
-20.57  -12.16749109
-21.72  -8.70333481
-22.87  -5.931814138
-24.06  -4.004803708
-25.23  -2.721246399

and 1:64 file:

x  y  
-18.13  -20.99567863
-19.29  -17.79263496
-20.43  -13.67778071
-21.24  -9.779891912
-22.73  -6.671620397
-23.88  -4.447331784
-25.36  -3
-25.78  -2.301029996

Both has negative values and my graph is not draw correctly.

The picture shows what I got. I added the black line with expected line and green colors represents the values of axis.

Thanks in advance for your time and ideas.

Answer 1

I think you are looking for the key 'x dir=reverse'. Without the reversed direction, -25 is left and -18 is right as in most plots.

\documentclass{article}
\usepackage{pgfplots}
\pgfplotsset{compat=1.3} %important, must be equal or higher than 1.3
\begin{document}
    \begin{tikzpicture}
    \begin{axis}[xlabel={Power (dBm)}, ylabel={log BER (---)},
    legend pos=north east, grid=major,
    x dir= reverse,
    axis x line* = top, %draw x axis line on top
    extra description/.code={\draw (0,0) -- (1,0);} %draw an extra line where the original x-axis was
    ]
    \addplot table[x=power,y=BER] {
        power   BER     
        -18.28  -20.00392635
        -19.42  -16.09799711
        -20.57  -12.16749109
        -21.72  -8.70333481
        -22.87  -5.931814138
        -24.06  -4.004803708
        -25.23  -2.721246399
    };
    \addplot table[x=power,y=BER] {
        power   BER
        -18.13  -20.99567863
        -19.29  -17.79263496
        -20.43  -13.67778071
        -21.24  -9.779891912
        -22.73  -6.671620397
        -23.88  -4.447331784
        -25.36  -3
        -25.78  -2.301029996
    };
    \addlegendentry{1:32}        
    \addlegendentry{1:64}
    \end{axis}
    \end{tikzpicture}
\end{document}