To see beyond 10 days, its a simple change to the php.
In the file admin/index.php, change this line (on/ near line 148) to limit a higher number than 10. Just change the 10 to whatever you want. Change nothing else.
The box heading will automatically be accurate: "Counter history for last xx recorded days."
$counter_query = "select startdate, counter, session_counter from " . TABLE_COUNTER_HISTORY . " order by startdate DESC limit 10";
$counter = $db->Execute($counter_query);
Backup first; backpedal as necessary.