Presumably when it says

$DB->get_field_sql('SELECT timeend FROM

{stats_user_daily} ORDER BY timeend');

It means

$DB->get_field_sql('SELECT MIN(timeend) FROM {stats_user_daily}

');

The attached patch fixes this. I guess the fix needs to be backported to 1.9 though.

Also, I don't think "Sorry, there is no available data to display" should be a print_error. It should be a nicer message than that. I have not fixed that.