How can I use two x axes in pgfplots?

Question

I would like to draw two x axes with ticks and legends in the bottom and top axes where one axis is a linear transformation of the other for the same y range. There is a similar application described in section 4.8.10 of the pgfplots manual for two y axes. Are two x axes possible using the axis environment?

Answer 1

As a default, both axes are plotted left/right & top/bottom. That’s why you get both ticks on the upper axis (I assume that is what you meant). You need to explicitly tell pgfplots you don’t want the upper part of the x-axis in the first {axis} by supplying axis x line*=bottom. Anyway, I find this really irritating to look at. Unless you supply additional information to the plot, there is no way of distinguishing which curve belongs to which axis, making the plot useless. Maybe in your application there is less chance of confusion, but still, beware.

\documentclass{article}

\usepackage{pgfplots}

\begin{document}
  \begin{tikzpicture}
    \begin{axis}[
      scale only axis,
      xmin=-5,xmax=5,
      axis y line*=left,
      axis x line*=bottom]
      \addplot {x^2};
    \end{axis}
    \begin{axis}[
      scale only axis,
      xmin=0,xmax=15,
      domain=0:15,
      axis y line*=right,
      axis x line*=top]
      \addplot[red] {x};
    \end{axis}
  \end{tikzpicture}
\end{document}