For openSUSE Tumbleweed (at least), the following runtime requirement
"perl-rrdtool" should be included in the package (spec) definition. Otherwise
errors like the following appear in the /var/log/monitorix logfile (and no
graph output occurs):

Thu Feb  7 23:27:27 2019 - WARNING: unable to load module 'system'. Can't
locate RRDs.pm in @INC (you may need to install the RRDs module) (@INC
contains: /usr/bin/lib /usr/lib/monitorix
/usr/lib/perl5/site_perl/5.28.1/x86_64-linux-thread-multi
/usr/lib/perl5/site_perl/5.28.1
/usr/lib/perl5/vendor_perl/5.28.1/x86_64-linux-thread-multi
/usr/lib/perl5/vendor_perl/5.28.1
/usr/lib/perl5/5.28.1/x86_64-linux-thread-multi /usr/lib/perl5/5.28.1
/usr/lib/perl5/site_perl) at /usr/lib/monitorix/system.pm line 26.
BEGIN failed--compilation aborted at /usr/lib/monitorix/system.pm line 26.
Compilation failed in require at (eval 9) line 1.
BEGIN failed--compilation aborted at (eval 9) line 1.
...