Re: empty data-files

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

Petr Mikulik <mikulik <at> physics.muni.cz> writes:
> This behviour is fine with me, just there should be an int_error("nothing to
> plot") if there was only a single (empty) file to plot. Please update your
> patch, add taking care of 3d plots.

In that case, you get this:
         warning: no valid data points found in specified file
         all points y value undefined!
Clear enough, isn't it?

> What should happen here:
>  plot x*x, 'empty.dat'

You get this:
         warning: no valid data points found in specified file
         x range is invalid
So you have to use, for example:
plot [-1:1] x*x, 'empty.dat'

The titles x*x and 'empty.dat' are shown, but only x*x is plotted. Not bad.

----------------------------------------------------------------------------
Ethan Merritt <merritt <at> u.washington.edu> writes:
> It's more complicated that that.  The first thing that will happen
> if you just change int_error into int_warn is likely to be a 
> divide-by-zero error as the autoscaling fails.

That doesn't happen. If an empy datafile is plotted alone, you get:
         warning: no valid data points found in specified file 
         all points y value undefined!

> Beyond that, there
> is the question of consistent behaviour of the plot.
> For instance, "skipping" a plot  should not change the color
> assignments of the remaining plots.

There is no change of color.
There is no "skipping" (the title of an empty plot is still there).

----------------------------------------------------------------------------
Hans-Bernhard Broeker <broeker <at> physik.rwth-aachen.de> writes:
> But the plot command, as given, still is wrong.  An entire dataset is 
> missing --- there'll consequently be all kinds of strange behaviour on the 
> plot, which are pretty much guaranteed to make the resulting diagram at 
> least hard to interpret, possibly unusable.

Only the points are missing. The title of each empty dataset is still there. So
the plot command is _not_ wrong.

> If so, then it's a grave bug in the generator that it didn't notice it
> just created a datafile without any data in it.

The bug is not in the generator but in gnuplot. An empty datafile is not wrong.

> That would cause a new problem: gnuplot would now try to generate a plot, 
> even if *none* of the files in the plot contain any data.

No plot is generated in that case. You just get this message:
         all points y value undefined!

And in fact, that behavior doesn't suit me: I would like gnuplot to generate a
plot with no point in it, but with a title and labels (the x and y ranges must
be set in that case of course). I use gnuplot to plot real time data, and I draw
only the data of the last minutes. If in that time there is no data at all, I
want to see the plot all the same (and I certainly don't want gnuplot to stop).

---------------------------------------------------------------------------
SUMMARY: I can't see any drawback in my small patch. But it is no enough. It
should be possible to plot an empty datafile (everything but the points).