Preprocess function to modify title

I have created a view (called magazine_articles) which generates a list of titles and authors (based on a content type called Article).
When I group the fields by IssueDate, the block appears thus:

Jan 2010
Title        Author
Title        Author

Nov 2009
Title       Author

etc

The articles are in a bimonthly magazine which is published on our website.
 
What I want to do is to convert the group headings to:

Jan-Feb 2010
Nov-Dec 2010
Sep-Oct 2010

I have tried to write a preprocess function in my template.php file (in my theme directory):

function faith_home_preprocess_views_view__magazine_articles(&$variables) {
 
  $month = substr($variables['title'],0,3);
  $year = substr($variables['title'],4,4);
switch($month){
    case "Jan": $issuename="Jan-Feb"; break;
    case "Mar": $issuename="Mar-Apl"; break;
    case "May": $issuename="May-Jun"; break;
    case "Jul": $issuename="Jul-Aug"; break;
    case "Sep": $issuename="Sep-Oct"; break;
    case "Nov": $issuename="Nov-Dec"; break;
    default: $issuename = "None"; break;
}
$title = $issuename." ".$year;
 
 
 
  $variables['magtitle'] = $title;


}

I have modified the coding in my views-view-table--magazine_articles.tpl.php file to:

<?php if (!empty($magtitle)): ?>
  <<?php print $group_element; ?><?php print drupal_attributes($group_attributes); ?>>
    <?php print $magtitle; ?>
  </<?php print $group_element; ?>>

but no group headings are printed.

Can anyone tell me how I can solve this?