Thank You very much. 

I still get the errors.
I rechecked my data and the data is perfect.

my data are large double numbers , say 
1.09005965871078 
0.585952618303551 
0.759547391474863 
0.585952618303551 
0.759547391474863 
0.585952618303551 
0.585952618303551 

so i tried the test program with similar data & i was not able to produce the graph. So could the error be cause of the data?

the error is :
Useless use of a constant in void context at /usr/lib/perl5/site_perl/5.8.0/GD/Graph/histogram.pm line 203.
Use of uninitialized value in string eq at /usr/lib/perl5/site_perl/5.8.0/GD/Graph/histogram.pm line 42.
Argument "" isn't numeric in numeric eq (==) at /usr/lib/perl5/site_perl/5.8.0/GD/Graph/histogram.pm line 181.
Use of uninitialized value in string eq at /usr/lib/perl5/site_perl/5.8.0/GD/Graph/histogram.pm line 42.
Use of uninitialized value in string eq at /usr/lib/perl5/site_perl/5.8.0/GD/Graph/histogram.pm line 42.