empty data-files

[Jacques B. <jac...@on...>]

When you try to plot an empty data-file with "plot", the output terminal 
is not updated, and you get this message:

"no valid data points found in specified file".

Why not.

But if you try to plot 2 data-files, one empty the other not, you get 
the same answer, and that is not OK. There _is_ something to plot. That 
situation may arise if the data-files and the gnuplot script are 
generated automatically.

So I think that in plot2d.c:

int_error(c_token, "no valid data points found in specified file");

should be replaced with:

int_warn(c_token, "no valid data points found in specified file");