PNP provides access to RRD data using the xport controller. The output format can be specified. At the moment the formats xml, json and csv are supported.
The controller can be called using the URL
/pnp4nagios/xport/<format>?host=<hostname>&srv=<servicedesc>
whereas <format> has to be replaced with the desired format.
You can also use wget to generate images and place them in periodic reports. One example may be:
wget -O image.png 'http://<user>:<pass>@<nagios-server>/pnp4nagios/image?host=<hostname>&srv=<service>&view=2&source=0'
view=<n> limits the graph to the timeperiod specified in config.php
source=<n> only shows one data source if you have more than one in your RRD file
Instead of view you can use start and/or end to specify the time period. For details please look at "time ranges".