So if you did that, you would dump out all the data for one row into your table, using PHP, and add in something like this:
<tr onmouseover="showMin(this,<?php echo $row['lowest'];?>)">
<td><?echo $row['tag'] ?></td>
... other fields in the row ...
function to display a nice popover box with the minimum price for that tag.
NOTE: Do you REALLY want this grouped by tag??? Wouldn't it make more sense to have it grouped by (say) leg1
? So you would have the lowest rate for all "BHM-MAD" flights? But that's up to you and a trivial change to the SQL query, of course.