Quiet down graph generating cron job on new installations.
Quiet down graph generating cron job on new installations.

The cron job complained about empty data files because there were
no requests of a given category used in the graph.  This change make
sure every data set have at least one zero value data point.

Based on a patch proposal from Gareth Rees.

https://github.com/mysociety/alaveteli/issues/619

Br3nda/alaveteli,Br3nda/alaveteli,nzherald/alaveteli,4bic/alaveteli,4bic/alaveteli,nzherald/alaveteli,nzherald/alaveteli,4bic/alaveteli,Br3nda/alaveteli,4bic/alaveteli,Br3nda/alaveteli,nzherald/alaveteli,4bic/alaveteli,Br3nda/alaveteli,nzherald/alaveteli