Line Char no zero values

I have generat a Line Char. Now I set one value to zero. The line graph disappear in certain sections, why? What is it for a chart if I put no value on zero

<?php
	$chart = array(
		'#chart_id' => 'test_chart',
		'#title' => chart_title(t('Servings'), 'cc0000', 15),
		'#type' => CHART_TYPE_LINE,
		'#size' => chart_size(400, 200),
		'#adjust_resolution' => TRUE,
	);

	$chart['#data']['months'] = array(1, 2, 0, 4, 2,6,7,8,9,1,0,12);


	//$chart['#legends'][] = t('Fruits');
	//$chart['#legends'][] = t('Meats');
	//$chart['#legends'][] = t('Dairy');

	$chart['#data_colors'][] = '00ff00';
	$chart['#data_colors'][] = 'ff0000';
	$chart['#data_colors'][] = '0000ff';

	$chart['#mixed_axis_labels'][CHART_AXIS_Y_LEFT][0][] = chart_mixed_axis_range_label(0, 5);
	//$chart['#mixed_axis_labels'][CHART_AXIS_Y_LEFT][1][] = chart_mixed_axis_label(t('Count'), 95);

	$chart['#mixed_axis_labels'][CHART_AXIS_X_BOTTOM][1][] = chart_mixed_axis_label(t('jan'));
	$chart['#mixed_axis_labels'][CHART_AXIS_X_BOTTOM][1][] = chart_mixed_axis_label(t('feb'));
	$chart['#mixed_axis_labels'][CHART_AXIS_X_BOTTOM][1][] = chart_mixed_axis_label(t('mar'));
	$chart['#mixed_axis_labels'][CHART_AXIS_X_BOTTOM][1][] = chart_mixed_axis_label(t('apr'));  
	$chart['#mixed_axis_labels'][CHART_AXIS_X_BOTTOM][1][] = chart_mixed_axis_label(t('mai'));
	$chart['#mixed_axis_labels'][CHART_AXIS_X_BOTTOM][1][] = chart_mixed_axis_label(t('jul'));
	$chart['#mixed_axis_labels'][CHART_AXIS_X_BOTTOM][1][] = chart_mixed_axis_label(t('jun'));
	$chart['#mixed_axis_labels'][CHART_AXIS_X_BOTTOM][1][] = chart_mixed_axis_label(t('aug'));
	$chart['#mixed_axis_labels'][CHART_AXIS_X_BOTTOM][1][] = chart_mixed_axis_label(t('sep'));
	$chart['#mixed_axis_labels'][CHART_AXIS_X_BOTTOM][1][] = chart_mixed_axis_label(t('okt'));
	$chart['#mixed_axis_labels'][CHART_AXIS_X_BOTTOM][1][] = chart_mixed_axis_label(t('nov'));
	$chart['#mixed_axis_labels'][CHART_AXIS_X_BOTTOM][1][] = chart_mixed_axis_label(t('dez'));
		  

  echo chart_render($chart);

?>