\\ Writes pairs of args and vals to a file,
\\ e.g. for plotting with pst-plot. If args is
\\ a number, args start  with this number;
\\ otherwise args has to be a vector. Attention:
t_writeplotpairs (file,vals,args=0,rowlen=20) = {
n=#vals; if (type(args)=="t_VEC",\
w=vector(2*n,i, if (i%2, args[i\2+1], vals[i\2])),
w=vector(2*n,i, if(i%2, i\2+args, vals[i\2])));
T=t_table(w,rowlen); t_write(file,T)}