pgfplots with 2 x axis, misalignment of upper x xticklabels

Question

I need a plot with two x-axes, on at the top the other at the bottom. I try to do this quite successfully with this example code:

\documentclass[12pt,twoside, a4paper]{report}
\usepackage[german]{babel}
\usepackage[T1]{fontenc}
\usepackage[latin1]{inputenc}  
 \usepackage{tikz} 
 \usepackage{pgfplots}
\pgfplotsset{compat=1.6}
\begin{document}


\begin{tikzpicture}
 %\pgfplotsset{set layers}%Not used
\begin{semilogyaxis}[filter discard warning=false,scale only axis,
 width=5.41cm,
  height=4.415cm, 
 xmin=-7.5,xmax=7.5,ymin=0.01,ymax=50, 
 axis x line*=bottom,
 xtick={-6,-4,-2,0,2,4,6},
  enlargelimits=false,axis on top=true,
  xlabel=lower x scale,ylabel=y scale]
 \addplot[color=red,mark=x] coordinates {%just fake data
 (-6,0.1) (0,10) (6,0.2)};
 \end{semilogyaxis}

 \begin{semilogyaxis}[filter discard warning=false,
  scale only axis,
   width=5.41cm,
   height=4.415cm, 
   xmin=-0.7409,xmax=0.7409,ymin=0.01,ymax=50, 
   minor tick num=3,
   axis x line*=top,
   axis y line=none,
   enlargelimits=false,axis on top=true,
   xlabel=upper x scale]
   \end{semilogyaxis}
  \end{tikzpicture}
  \end{document}

The result is quite nice, as you can see here:

But due to the '-0.5' on the upper left corner there is a slight misalignment of the labels. How can I get rid of this problem?

Answer 1

The minus sign has a greater text depth than the other characters in the labels, causing the misalignment. You can set text depth=0pt to fix this:

\documentclass[12pt,twoside, a4paper]{report}
\usepackage[german]{babel}
\usepackage[T1]{fontenc}
\usepackage[latin1]{inputenc}  
 \usepackage{tikz} 
 \usepackage{pgfplots}
\pgfplotsset{compat=1.6}
\begin{document}


\begin{tikzpicture}
 %\pgfplotsset{set layers}%Not used
\begin{semilogyaxis}[filter discard warning=false,scale only axis,
 width=5.41cm,
  height=4.415cm, 
 xmin=-7.5,xmax=7.5,ymin=0.01,ymax=50, 
 axis x line*=bottom,
 xtick={-6,-4,-2,0,2,4,6},
  enlargelimits=false,axis on top=true,
  xlabel=lower x scale,ylabel=y scale]
 \addplot[color=red,mark=x] coordinates {%just fake data
 (-6,0.1) (0,10) (6,0.2)};
 \end{semilogyaxis}

 \begin{semilogyaxis}[filter discard warning=false,
  scale only axis,
   width=5.41cm,
   height=4.415cm, 
   xmin=-0.7409,xmax=0.7409,ymin=0.01,ymax=50, 
   xticklabel style={text depth=0pt},
   minor tick num=3,
   axis x line*=top,
   axis y line=none,
   enlargelimits=false,axis on top=true,
   xlabel=upper x scale]
   \end{semilogyaxis}
  \end{tikzpicture}
  \end{document}