=== Hyperref and Tikz Externalize ===

Tikz Externalize does not work well with Hyperref without some extra measures. In order to get around the problem, include the following lines in your preamble

{{{
   \tikzifexternalizing{%
      \renewcommand{\hypersetup[1]}{}
   }{%
      \usepackage{hyperref}  
   }
   \hypersetup{
      colorlinks,           % These are of course only example options.
      bookmarksnumbered,
      linkcolor={black},
      citecolor={black},
      urlcolor={blue!90!black}
    }
    % \hypersetup{draft}    % Remove the comment character for printing. 

}}}