Null and 0 on BarGraph

The code below is the source for this graph.

<?php
// Null and 0 on BarGraph
require_once 'SVGGraph/autoloader.php';

$settings = array(
  'auto_fit' => true,
  'back_colour' => '#eee',
  'back_stroke_width' => 0,
  'back_stroke_colour' => '#eee',
  'stroke_colour' => '#000',
  'axis_colour' => '#333',
  'axis_overlap' => 2,
  'grid_colour' => '#666',
  'label_colour' => '#000',
  'axis_font' => 'Arial',
  'axis_font_size' => 10,
  'pad_right' => 20,
  'pad_left' => 20,
  'project_angle' => 45,
  'minimum_grid_spacing' => 20,
  'show_subdivisions' => true,
  'show_grid_subdivisions' => true,
  'grid_subdivision_colour' => '#ccc',
);

$width = 300;
$height = 200;
$type = 'BarGraph';
$values = ['One' => 1, 'Null' => null, 'Four' => 4, 'Zero' => 0, 'Five' => 5];

$colours = [ ['red', 'yellow'], ['blue', 'white'] ];

$graph = new Goat1000\SVGGraph\SVGGraph($width, $height, $settings);
$graph->colours($colours);

$graph->values($values);
$graph->render($type);

Please note: the source code above comes from a script used to generate one of the example graphs on the site. This is simply a wrapper page to apply highlighting and make the source code easier to read, so there is no content here to describe what the source code is doing.