//Inspired by a good idea of Mr Jens Schwaiger
size(8cm,0);

string format_(string s, real x, string oldsep=",", string newsep="{,}"){
  string s_=format(s, x);
  if(newsep!=oldsep) s_=replace(s_, oldsep, newsep);
  return s_;
}

string format(string, real) = new string(string format, real x)
{
  return format_(format,x);
};

string format(real) = new string(real x)
{
  return format_(defaultformat,x);
};

include graph;  

xlimits( -3pi, 3pi);  

xaxis(BottomTop(), 
      Ticks(Label("$%.2f$",red),
            Step=2pi,step=pi/5,pTick=.8red));