InvoicePlane get total in reports by year

<!DOCTYPE html>

<?php // definimos previamente las 3 variables que suman los totales

$report_inv_count = '';
$report_total = '';
$report_total_tax = '';

    ?>

<html lang="<?php echo trans('cldr'); ?>">
<head>
    <title><?php echo trans('sales_by_client'); ?></title>
    <link rel="stylesheet" href="<?php echo base_url(); ?>assets/<?php echo get_setting('system_theme', 'invoiceplane'); ?>/css/reports.css" type="text/css">
</head>
<body>

<h3 class="report_title">
    <?php echo trans('sales_by_client'); ?><br/>
    <small><?php echo $from_date . ' - ' . $to_date ?></small>
</h3>

<table>
    <tr>
        <th><?php echo trans('client'); ?></th>
        <th class="amount"><?php echo trans('invoice_count'); ?></th>
        <th class="amount"><?php echo trans('sales'); ?></th>
        <th class="amount"><?php echo trans('sales_with_tax'); ?></th>
    </tr>
    <?php foreach ($results as $result) { ?>
        <tr>
            <td><?php _htmlsc(format_client($result)); ?></td>
            <td class="amount"><?php echo $result->invoice_count; $report_inv_count = $result->invoice_count + $report_inv_count; ?></td>
            <td class="amount"><?php echo format_currency($result->sales); $report_total = $result->sales + $report_total; ?></td>
            <td class="amount"><?php echo format_currency($result->sales_with_tax); $report_total_tax = $result->sales_with_tax + $report_total_tax; ?></td>

        </tr>
    <?php } ?>

    <?php if(isset($report_inv_count) && isset($report_total) && isset($report_total_tax) ){ ?>

            <tr>
                <td><?php echo '<b>Total</b>'; ?></td>
                <td class="amount"><?php echo '<b>'.$report_inv_count.'</b>'; ?></td>
                <td class="amount"><?php echo '<b>'.format_currency($report_total).'</b>'; ?></td>
                <td class="amount"><?php echo '<b>'.format_currency($report_total_tax).'</b>'; ?></td>
            </tr>

            <?php } ?>

</table>

</body>
</html>