Help: how many rows were returned?

[Shao, Chunning]

I need to plot a result set from DBI.  Before I can do that, I need to know how many 
rows are returned, so I can set up my x, y axis right.

I am doing
#######################################################
$csr = $dbh->prepare($sql);
$csr->execute();

#get the number that how many rows returned
my $number_rows = 0;
while (my @rows = $csr->fetchrow_array )
{
        $number_rows +=1;
}
Set my x,y axis accordingly............

#begin to plot the data
my ($x_definition,$y_definition);
while (my @row = $csr->fetchrow_array )
{
        $counter = 0;
        foreach my $data (@row)
          Plot the data
}
########################################################
but I got an error 
281
DBD::Oracle::st fetchrow_array failed: ERROR no statement executing (perhaps you need 
to call execute first) at ./test.pl line 162.

Do I have to make a second db call, or there is better way of doing it?

Thanks

chunning