#26 vector without arrowhead

[Gavin Salam]

Would it be possible to have an additional style
like vector, except without the arrowhead (or
alternatively an option to vector that allows one to
set the arrowhead size to zero).

This would make it possible for a user to construct
arbitrary styles (e.g. I need errorbars which have
ticks part of the way up rather than at the
extremities, so as to indicate statistical errors as
opposed to total statistical+systematic errors, and
would construct this from a normal errorbar with a
"headless" vector extending vertically beyond the
normal errorbar).

Looking at the code, it seems like it would be fairly
painless to implement as a new style. But perhaps I'm
missing something and the facility is already
available?

[Hans-Bernhard Broeker]

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I'm not quite convinced such a style really would be useful.
You can always achieve the same goal with a transformed
datafile. I.e. for each headless vector, instead of a
single line

x1 y1 x2 y2

you would have *three* lines in the datafile:

--- begin ---
x1 y1
x2 y2

--- end ----

The blank line is important. Plot this 'with lines' and you
get one line segment for each repetition of this pattern,
from (x1,y1) to (x2, y2).

[Gavin Salam]

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Yes, I'm aware of the way of drawing obtaining
segments with the "line" style and blank lines in
the data file, though I hadn't thought of using it
in this context.

Having said that, if I want my data points to look
(schematically) like the following

y+yerr2
|
--- y+yerr1
|
* y
|
--- y-yerr1
|
y-yerr2

x

(widely used in some branches of physics, often
also with asymmetric error bars, see
e.g. http://arxiv.org/abs/hep-ex/0205026\), then
it's necessary either

. to have two separate data files, one for a
normal "errorbar style" (x:y:yerr1) and a second
one for all the line segments
(x,y-yerr2:x,y+yerr2)

or

. play tricks with a single data file, e.g.

--- begin ---
x y yerr1 (y-yerr2)
x y yerr1 (y+yerr2)

--- end ---
and then do
> plot "file" u 1:2:3 w e, "file" u 1:4 w l

in which case one can't e.g. then have (in the
same datafile) joined lines going through all
the data points (without grepping out the blank
lines...)

Both these approaches are perfectly feasible, but
a little bit messy. So some other way of doing it
(ideally without having to modify the datafile)
would be nice. It doesn't necessarily have to be a
vector without an arrowhead (I appreciate that
introducing too many styles will just confuse
people). Being able to set different widths for
the tics in each plot command, instead of a global
"set bars" option would be just as good.

The (only ?) advantage of a headless vector is
that it would make it possible to construct other
more complex styles from combinations of gnuplot
commands rather than having to write special
datafiles (or bother the gnuplot developers :-)
for each one.

[Hans-Bernhard Broeker]

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For that particular purpose, I'd (ab)use the 'with
boxxyerrorbars' style:

set style line 1 lt 3 pt 5
plot 'datafile' u 1:2:3 w err ls 1, \
     '' u 1:2:\(0\):4 w boxxyerr ls 1

And as far as advantages by not haveing to doctor special
datafiles for general line drawing usages --- I don't see
any. The only difference would be whether you doctor based
on datapoints or on series of datapoints. Actually, I'd say
that 'with lines' and a properly doctored datafile would be
preferrable, because you don't have to repeat datapoints in
the file in order to connect them.

[Gavin Salam]

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The "abuse" of the 'with boxxyerrorbars' style is good
enough!

Thank you for your help.

[Harald Harders]

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Now, there is a patch called "arrow styles via 'set style
arrow'" which allows this directly:

plot ... with vector nohead

But you will need a current cvs version of gnuplot.