Diagonal lines in commutative diagrams

Question

Consider

\begin{document}

 \usepackage{tikz-cd}

 \begin{tikzcd}
  T \arrow[r, "\eta T"] \arrow[swap, d, "T \eta" red ] 
  & T^2 \arrow[d, "\mu" red] \\
  T^2 \arrow[swap, r, red, "\mu" blue]
  & T 
  \end{tikzcd}

  \end{document}

How can I draw a diagonal arrow from the top left node to the bottom right node? In addition, how can I draw an arrow parallel to this arrow? I looked in the documentation for the package but could not find the answer.

Answer 1

Here it is, with the simplified syntax and nicer arrow tips (in my opinion…):

\documentclass[a4paper, 12pt]{article}
\usepackage{tikz-cd}

\begin{document}

\begin{tikzcd}[ arrows={-stealth}]
  T \rar["\eta T"] \dar[swap, "T \eta" red ]\drar[shift left]\drar[shift right]%
  & T^2 \dar["\mu" red] \\
  T^2 \rar[swap, red, "\mu" blue]
  & T
\end{tikzcd}

\end{document}