history file truncated to two lines

I have inherited a webalizer installation on a large system which was migrated from one host to another. Webalizer has over 10 years of history on this system, but since the migration, the history file self truncates each month so that the only non-zero values are in the last two months. This happens whether the configuration index/graph history values are left as defaults or set explicitly (they are both currently set to the maximum).

Everything else seems fine - the new month files are generated incrementally on a daily basis, and this all works correctly.

I have two questions:

1. Has anyone seen this behaviour before or know where to look to fix it?
2. I have no backup for the history file so no values for the last two years, other than the last two months. Is there any way to regenerate the values other than writing a script to parse the html pages for the missing months?

Webalizer version 2.2.3
Ubuntu 14.04.2

Thanks

Graham