Arrows are not drawn

[jjatria]

This might be a problem between the keyboard and chair, but if so, I'd say it suggests an issue with the documentation.

After reading https://metacpan.org/pod/PDL::Graphics::Gnuplot#arrow-place-an-arrow-or-callout-line-on-the-plot I attempted to draw an arrow on a plot, but nothing was drawn. I can get the similar label to work as expected, but not arrow. See below for an illustration:

use PDL;
use PDL::Graphics::Gnuplot;

my $win = gpwin;
$win->plot(
    {
        # This works
        label => [
            [
                'start',
                at => [ 25, 'graph(0.5)' ],
                'center',
            ],
            [
                'end',
                at => [ 75, 'graph(0.5)' ],
                'center',
            ],
        ],

        # This doesn't
        arrow => [
            [
                from => [ 25, 'graph(0.5)' ],
                to   => [ 75, 'graph(0.5)' ],
            ],
        ],
    },
    sequence(100) ** 2,
);

$win->pause_until_close;

This is the resulting plot, with the labels but no arrows:

Is this a bug in PDL::Graphics::Gnuplot? Or am I missing something?

[zmughal]

The positions there are given as strings. Here are multiple arrows.

$win->plot(
    {
        label => [
            [
                'start',
                at => [ 25, 'graph(0.5)' ],
                'center',
            ],
            [
                'end',
                at => [ 75, 'graph(0.5)' ],
                'center',
            ],
        ],

        arrow1 => [
                 from =>  '25,graph(0.5)',
                 to   =>  '75,graph(0.5)',
        ],

        arrow2 => [
                 from =>  'graph(0),graph(0)',
                 rto   =>  'graph(1),graph(1)',
        ],
    },
    sequence(100) ** 2,
);

[mohawk2]

Does that mean this is just a documentation issue? Does someone feel like PR-ing a doc update?

[jjatria]

The workaround is very useful, thanks @zmughal! I don't really understand why PDL::Graphics::Gnuplot seems capable of understanding positions as array references when using labels (and a couple of other elements) but not when using arrows. This inconsistency seems like maybe the symptom of some underlying bug?

Likewise, it's odd to me that you can do

label => [
    [ # one label ... ],
    [ # another label ... ],
]

But you have to do

arrowfoo => [
    # one arrow...,
],
arrowbar => [
    # another arrow...,
]

[mohawk2]

@jjatria That's no moon! I mean workaround. It's the documented (since forever) behaviour for arrow.

If you'd like to make arrow work like label, both are defined in $pOptionsTable:

PDL-Graphics-Gnuplot/lib/PDL/Graphics/Gnuplot.pm

Lines 4928 to 4929 in 9fd58b6

	'arrow' => ['N','N',undef,undef,
	'allows specification of arrows to be drawn on subsequent plots' ],

PDL-Graphics-Gnuplot/lib/PDL/Graphics/Gnuplot.pm

Lines 4990 to 4991 in 9fd58b6

	'label' => ['N','NL',undef,undef,
	'Define text labels to be rendered in plot (numeric index; see docs)' ],

If you want to make a PR that adds arrows, go ahead! We should probably leave the other scheme in place (with probably a clash-avoiding special case since arrow* matches arrows) - but I'm open to the idea of just removing arrow as was, if we can show it's not used in P:G:G's reverse deps.