bugfix: make sure the plot start exactly at $start_date
bugfix: make sure the plot start exactly at $start_date

cherry picked from commit master - bcd6efd7 and bugfixed
again to correctly initialize the first data-point

Ensembl/ensembl-hive,Ensembl/ensembl-hive,Ensembl/ensembl-hive,Ensembl/ensembl-hive