plot_t<-function(fun, xlim_pi=c(0, 2), by=1){
if(length(xlim_pi)!=2)print("xlim_pi, must be 2 constants")
a<-xlim_pi[1]; b<-xlim_pi[2]
xlab<-paste(by, "pi")
ylab<-deparse1(fun)
plot(fun, xlim=c(a*pi, b*pi), xaxt="n", xlab="pi", ylab=ylab)
lab<-paste(lab<-seq(a, b, by=by), "pi")
axis(side=1, at=seq(a*pi, b*pi, by=by*pi), label=lab)
}