Perl - GD::Graph - Gap between plot and y-axis - perl

Question

I am using Perl's GD::Graph
module to plot a signal profile. I've managed to get almost everything to look just about right, except that for some reason there is an irritating and unwanted gap between the y-axis and the first plotted point. How can I eliminate this?
Here is the code for the test page I've written (it display on my localhost server, but I tried installing it on my live server so that others here could see the result, it errors - I think there must be some Perl modules missing from my live site):
#!perl
use POSIX;
use CGI ':standard';
use GD::Graph::area;
use GD::Graph::colour qw(:colours);
use strict;
# Graph constants
GD::Graph::colour::add_colour( axes => [ 0x00, 0x00, 0x00 ] );
GD::Graph::colour::add_colour( earth => [ 0x00, 0x00, 0x00 ] );
GD::Graph::colour::add_colour( beam => [ 0x00, 0x00, 0x00 ] );
GD::Graph::colour::add_colour( clutter => [ 0x66, 0x33, 0x00 ] );
GD::Graph::colour::add_colour( terrain => [ 0x00, 0x99, 0x00 ] );
GD::Graph::colour::add_colour( earth => [ 0x00, 0x00, 0x00 ] );
GD::Graph::colour::add_colour( fre1 => [ 0xFF, 0x33, 0x00 ] );
GD::Graph::colour::add_colour( fre2 => [ 0xFF, 0x66, 0x00 ] );
GD::Graph::colour::add_colour( sky1 => [ 0x33, 0x66, 0x99 ] );
GD::Graph::colour::add_colour( sky2 => [ 0x66, 0x99, 0xCC ] );
# All arrays should same number of entries.
my #data = (
[ 0, undef, undef, undef, undef, undef, undef, undef, undef, undef, undef, undef, undef, undef, undef, 5, undef, undef, undef, undef, undef, undef, undef, undef, undef, undef, undef, undef, undef, undef, 10, undef, undef, undef, undef, undef, undef, undef, undef, undef, undef, undef, undef, undef, undef, 15, undef, undef, undef, undef, undef, undef, undef, undef, undef, undef, undef, undef, undef, undef, 20, undef, undef, undef, undef, undef, undef, undef, undef, undef, undef, undef, undef, undef, undef, 25, undef, undef, undef, undef, undef, undef, undef, undef, undef, undef, undef, undef, undef, undef, 30, undef, undef, undef, undef, undef, undef, undef, undef, undef ],
[ 450, 458, 459, 458, 457, 455, 453, 451, 449, 447, 444, 441, 438, 435, 432, 429, 426, 422, 420, 416, 413, 409, 406, 402, 399, 395, 391, 388, 384, 381, 376, 373, 369, 365, 361, 358, 353, 350, 345, 341, 337, 333, 329, 325, 321, 316, 313, 308, 304, 299, 296, 291, 286, 282, 277, 273, 268, 265, 259, 256, 250, 247, 241, 236, 232, 227, 223, 217, 213, 208, 204, 198, 194, 188, 184, 178, 173, 168, 163, 158, 152, 148, 142, 137, 131, 126, 120, 115, 109, 102, 97, 90, 85, 78, 72, 64, 58, 50, 43, 32 ],
[ 450, 453, 451, 449, 447, 444, 441, 438, 435, 432, 428, 426, 422, 418, 415, 411, 408, 404, 400, 396, 393, 389, 386, 381, 378, 374, 369, 366, 361, 358, 353, 350, 345, 342, 337, 334, 329, 326, 321, 316, 313, 308, 305, 300, 296, 291, 288, 283, 279, 274, 271, 266, 261, 257, 252, 249, 244, 240, 235, 231, 226, 222, 217, 212, 208, 203, 199, 194, 190, 185, 181, 176, 172, 166, 162, 157, 151, 147, 142, 138, 132, 128, 123, 118, 113, 108, 103, 98, 92, 87, 82, 76, 71, 65, 60, 54, 49, 42, 36, 27 ],
[ 450, 445, 440, 436, 432, 428, 423, 419, 414, 411, 406, 402, 397, 392, 389, 384, 380, 375, 372, 367, 363, 358, 354, 350, 346, 341, 336, 332, 328, 324, 319, 315, 310, 307, 302, 298, 293, 290, 285, 280, 276, 271, 268, 263, 259, 254, 250, 246, 242, 237, 233, 228, 224, 220, 215, 211, 206, 203, 198, 194, 189, 186, 181, 176, 172, 167, 164, 159, 155, 150, 147, 142, 138, 133, 129, 125, 120, 116, 111, 107, 102, 99, 94, 90, 85, 82, 77, 73, 68, 63, 60, 55, 51, 46, 43, 38, 34, 29, 25, 21 ],
[ 450, 437, 429, 423, 416, 411, 405, 400, 394, 389, 383, 379, 373, 367, 363, 357, 353, 347, 343, 337, 333, 328, 323, 318, 314, 309, 303, 299, 294, 290, 285, 281, 275, 272, 266, 262, 257, 253, 248, 243, 239, 234, 231, 226, 222, 217, 213, 208, 205, 200, 196, 191, 186, 183, 178, 174, 169, 166, 161, 157, 153, 149, 144, 140, 136, 132, 128, 124, 120, 116, 112, 108, 104, 100, 97, 92, 88, 84, 80, 77, 73, 69, 65, 62, 58, 55, 51, 48, 44, 40, 37, 34, 31, 27, 25, 22, 19, 17, 15, 14 ],
[ 450, 432, 422, 415, 406, 400, 393, 387, 380, 375, 368, 363, 357, 350, 345, 339, 334, 328, 324, 318, 313, 307, 303, 297, 293, 287, 281, 277, 271, 267, 262, 258, 252, 248, 243, 239, 233, 229, 224, 219, 215, 210, 206, 201, 197, 192, 188, 183, 180, 175, 171, 166, 161, 158, 153, 149, 145, 141, 136, 133, 128, 125, 120, 116, 112, 108, 105, 100, 97, 93, 89, 85, 82, 78, 75, 71, 67, 64, 60, 57, 53, 50, 46, 43, 40, 37, 34, 31, 28, 25, 22, 19, 17, 15, 13, 11, 10, 8, 8, 9 ],
[ 387, 344, 266, 240, 222, 215, 190, 156, 120, 76, 99, 151, 167, 158, 167, 158, 157, 148, 114, 89, 87, 28, 11, 11, 12, 12, 12, 13, 13, 13, 13, 23, 15, 34, 39, 38, 72, 107, 122, 134, 128, 128, 121, 100, 82, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16, 15, 15, 20, 20, 15, 15, 15, 15, 14, 14, 14, 14, 60, 53, 13, 13, 12, 12, 12, 12, 11, 11, 11, 10, 10, 9, 9, 9, 8, 8, 7, 7, 6, 6, 5, 5, 4, 4, 3, 2, 2, 1, 26 ],
[ 386, 343, 261, 235, 217, 210, 185, 151, 105, 61, 84, 136, 152, 143, 152, 153, 152, 143, 109, 84, 82, 23, 11, 11, 12, 12, 12, 13, 13, 13, 13, 23, 15, 34, 34, 33, 67, 102, 117, 129, 123, 128, 121, 100, 82, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16, 15, 15, 15, 15, 15, 15, 15, 15, 14, 14, 14, 14, 60, 53, 13, 13, 12, 12, 12, 12, 11, 11, 11, 10, 10, 9, 9, 9, 8, 8, 7, 7, 6, 6, 5, 5, 4, 4, 3, 2, 2, 1, 26 ],
[ 0, 1, 1, 2, 3, 3, 4, 4, 5, 5, 6, 6, 7, 7, 8, 8, 9, 9, 9, 10, 10, 11, 11, 11, 12, 12, 12, 12, 13, 13, 13, 14, 14, 14, 14, 14, 15, 15, 15, 15, 15, 15, 15, 15, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16, 15, 15, 15, 15, 15, 15, 15, 15, 14, 14, 14, 14, 13, 13, 13, 13, 12, 12, 12, 11, 11, 11, 10, 10, 10, 9, 9, 8, 8, 8, 7, 7, 6, 6, 5, 5, 4, 4, 3, 2, 2, 1, 1 ]
);
my $dist = 32.745;
my $mygraph = GD::Graph::area->new( 800, 300 );
$mygraph->set(
transparent => 0,
l_margin => 3,
b_margin => 3,
t_margin => 5,
r_margin => 5,
text_space => 3,
axis_space => 7,
tick_length => -5,
accent_treshold => 100,
x_label_position => 0.5,
bgclr => 'white',
fgclr => 'black',
boxclr => 'sky1',
labelclr => 'black',
axislabelclr => 'black',
textclr => 'black',
x_label => 'Kilometres',
y_label => 'Metres',
x_label_skip => 15.27,
x_last_label_skip => 1,
x_min_value => 0,
x_max_value => $dist,
borderclrs => [qw(fre2 fre1 black fre2 sky1 clutter terrain axes)],
dclrs => [qw(fre2 fre1 fre1 fre2 sky1 clutter terrain axes)]
) or warn $mygraph->error;
my $myimage = $mygraph->plot( \#data ) or die $mygraph->error;
print "Content-type: image/png\n\n";
print $myimage->png;
Background Information
I currently have a very successful web page which enables UK citizens to align a terrestrial TV aerial towards a given transmitter.
Here's an example of it in action
Once loading has completed, and the relevant buttons enabled, there three available visual components to help align the aerial and predict the likely usefulness of the signal:
A Google map for finding a landmark to point the aerial towards
A UK Ordnance Survey map showing the signal path
A vertical signal profile showing potential obstructions
It is the last of these, which is obtained by pressing the Signal Profile button at the bottom of the form and is displayed at the bottom of the page beneath the two maps, that is of concern here. I'm looking to replace it because it currently uses the Google Chart Tools API which unfortunately has been deprecated by Google and was supposed to end in 2015, but fortunately so far they've kept it running.
I've been looking ever since for a replacement come the day that Google throw the switch, but have had very limited success. If anyone knows of a package I could install on my own server to replace Google's chart drawing functionality, I'd be most grateful to know about it.

Answer 1

This feels like an off-by-one error in the code, but I don't have time to investigate further now. I'll just note that a work-around seems to be to replace:
x_min_value => 0,
with
x_min_value => 1,