[plugin nimble portfolio] Meta box checkbox

Hi,
I know that, this issue was hundrets of times on furum. I’m still cant do this in my plugin.

This is a code:

<?php

function nimble_portfolio_create_section() {
    add_meta_box('nimble-portfolio-section-options', __('Opcje', 'nimble_portfolio_context'), 'nimble_portfolio_section_options', 'portfolio', 'side', 'high');
}

add_action('admin_menu', 'nimble_portfolio_create_section');

function nimble_portfolio_section_options() {
 global $post;
$meta_box_value = get_post_meta($post->ID, $mydata['nimble-portfolio_show_c'] , true);  

    ?>

<ul>
<li>
			<label for="nimble_portfolio"><?php _e('Cena: ', 'nimble_portfolio_context')?></label>
			<input type="text" id="nimble_portfolio" name="nimble_portfolio" value="<?php echo htmlspecialchars(nimble_portfolio_get_meta('nimble-portfolio')); ?>" />
			<span><?php _e('z?')?></span>
		</li>
<li>
			<label for="nimble_portfolio_time"><?php _e('Czas: ', 'nimble_portfolio_context')?></label>
			<input type="text" id="nimble_portfolio_time" name="nimble_portfolio_time" value="<?php echo htmlspecialchars(nimble_portfolio_get_meta('nimble-portfolio_time')); ?>" />
		</li>
<li>
			<?php if($meta_box_value == "yes") $checked = 'checked="checked"';?>
			<label for="nimble_portfolio_show_c"><?php _e('Poka? w cenniku: ', 'nimble_portfolio_context')?></label>
			<input type="checkbox" id="nimble_portfolio_show_c" name="nimble_portfolio_show_c" value="yes" <?php echo $checked ?>/>
		</li>
<li>
			<label for="nimble_portfolio_show_u"><?php _e('Poka? w us?ugach: ', 'nimble_portfolio_context')?></label>
			<input type="checkbox" id="nimble_portfolio_show_u" name="nimble_portfolio_show_u" value="<?php echo htmlspecialchars(nimble_portfolio_get_meta('nimble-portfolio_show_u')); ?>" />
		</li>
</ul>
	<input type="hidden" name="nimble_portfolio_noncename" id="nimble_portfolio_noncename" value="<?php echo wp_create_nonce(plugin_basename(__FILE__)); ?>" />
    <?php
}

add_action('save_post', 'nimble_portfolio_save_data', 1, 2);
function nimble_portfolio_save_data($post_id, $post) {

    // verify this came from the our screen and with proper authorization,
    // because save_post can be triggered at other times
    if (!wp_verify_nonce($_POST['nimble_portfolio_noncename'], plugin_basename(__FILE__)))
        return $post->ID;

    if ($post->post_type == 'revision')
        return; //don't store custom data twice

    if (!current_user_can('edit_post', $post->ID))
        return $post->ID;

    // OK, we're authenticated: we need to find and save the data
    // We'll put it into an array to make it easier to loop though.
    $mydata = array();
    $mydata['nimble-portfolio'] = $_POST['nimble_portfolio'];
	$mydata['nimble-portfolio_time'] = $_POST['nimble_portfolio_time'];
	$mydata['nimble-portfolio_show_c'] = $_POST['nimble_portfolio_show_c'];
	$mydata['nimble-portfolio_show_u'] = $_POST['nimble_portfolio_show_u'];

    // Add values of $mydata as custom fields
    foreach ($mydata as $key => $value) { //Let's cycle through the $mydata array!
        update_post_meta($post->ID, $key, $value);
        if (!$value)
            delete_post_meta($post->ID, $key); //delete if blank
    }
}
?>