sub set_column_widths {
    my $h         = shift; # reference to hash table of speeds, keyed by datetime, then by database
    my $databases = shift; # reference to hash of database names, where we will set the width values
    my $names     = shift; # ref to array of database names
                           #     (so we don't need to call keys on $databases repeatedly)
    for my $key (keys %$h){
        for my $db (@$names){
            my $l = length $h->{$key}{$db};  # get length of this item in the hash table
                                             # set column width to the widest length
            $databases->{$db} = $databases->{$db} > $l ? $databases->{$db} : $l;  
        }
    }
    for my $db (@$names){  # check the width of the database names themselves too
        my $l = length $db;
        $databases->{$db} = $databases->{$db} > $l ? $databases->{$db} : $l;  
    }
}