With PNP4Nagios, your custom template can be used to define all the graph definition -- with the exception of the time window, which is added to the parameter lists in $opt[] and $def[]. So, you cannot easily override the time window 'end' as it is already defined as 'now' by PNP4Nagios (and the 'start' is already defined relative to the end, based on the time range you selected in the web interface). In fact, RRDTool is fairly robust, so if it sees a start/end being redefined the last such definition usually takes precedence... but this doesn't solve your issue.
I think what you're trying to do is have the 1day graph (which normally starts at 'end-1day' and ends at 'now') to go from 'now-1day' to 'now+1day' so that your prediction line could fill the second part. This would need to be done by editing the PNP4Nagios code, which is a bit out of scope of this answer.
PNP4Nagios allows definition of the standard timeranges in the config.php; you can also define new timeranges when you call the graph. This means you can achieve the required time window like this:
pnp4nagios/graph?host=<hostname>&srv=<servicedesc>&start=-1day&end=+1day
... although this is just a one-off and does not override the defaults.
The current view config in PNP4Nagios does not allow the default views to specify an end offset, only a start offset.