Re: multiple plot lines with plot '-'

[Tait <gnu...@t4...>]

> > f(x,y,z)=<some function>
> > g(x)=<some other function>
> > plot 'datafile' using (column(f($1,$2,$3))):(column(g($4)), \
> > 	'data2file' using ($1>$2 ? column(g($4)) : 1/0)
>
> I think your example needs to be stripped down to the bare essentials of
> making the point you are trying to make. I'll have a guess about what
> you are trying to say, sorry if I miss the mark, and I think you are
> mistaken.

I was trying to make two separate points in that example, and you missed
the first -- and more important -- of them. To simplify:

plot 'datafile' using (column($1))

The column to be used for data is the column specified in the first
column of the data file. This is not and cannot be known until the file
(that column, at least) is actually read. The point of including the
functions is that we might not even know based on a single line of the
input what column we would need to preserve. The only way to know what
must be preserved is to actually evaluate the using conditions on the
input, see what results, and then reread the file for the next plot
because it, too, may have the exact same dereferencing behavior that
the first using statement had. And even more so, to the extent the first
using evaluation had side-effects that alter the meaning of the second
using statement, these side-effects cannot be anticipated ahead-of
time without actually doing the calculations demanded by the user.
These kinds of side effects are already suggested by demos like
http://gnuplot.sourceforge.net/demo_4.6/running_avg.html.

In short, the current behavior of gnuplot makes it impossible to
do a single-pass read of the data file. We could break the current
behavior, of course, but then we explicitly decrease the flexibility
we have for the sake of introducing other inconsistent and backward-
incompatible magic behavior.

At risk of repeating myself, caching may be worthwhile, but not in
the way and with the syntax you've suggested.

> >			 ... The behavior of '-' is currently the same as
> > for a data file. ...
>
> No, you need to read up on how '-' works.  This has been covered in a
> number of posts in this thread.

I know exactly how it works, and it works just like data files work.
I use and rely on this behavior. The behavior you want, and that
you (incorrectly, imo) describe as being the same as what's done for
data files is different. When you say plot 'file1', '', gnuplot does
not save 'file1' and rewind through this cache to find the data it
needs when plotting the ''. It plots 'file1', then when it sees ''
it reads 'file1' (again). When gnuplot plots '-', it reads the inline
data, and when it sees '' after, it reads the inline data again, not
a cached copy that it rewinds and replays. The fact that '-' might be
different on the first and second read is exactly the same as 'file1'
might be different on the first and second read. This describes the
same behavior for both 'file1' and '-'. What you're asking for is that
unlike the 'file1' case, '-' be handled differently, so that it saves
or caches the data and rewinds back and forth through this cache
instead of reading anew. This sort of special-case handling of '-'
might be a worthwhile option, but it's a separate feature, and one
that must be kept optional so as to not break backward compatibility
and important use cases for some users (like me).