Add API endpoint to get service graphs (#15138)

* Add API endpoint to get service graphs

* Docs: Add missing title to get_graph_by_service

doc/API/Devices.md

@@ -564,6 +564,38 @@ Output:
 
 Output is an image.
 
+### `get_graph_by_service`
+
+Get the graph for a service
+
+Route: `/api/v0/devices/:hostname/services/:service_id/graphs/:datasource`
+
+- hostname can be either the device hostname or id
+- service id
+- datasource is the name of the service datasource
+
+Input:
+
+- from: This is the date you would like the graph to start - See
+  [http://oss.oetiker.ch/rrdtool/doc/rrdgraph.en.html](http://oss.oetiker.ch/rrdtool/doc/rrdgraph.en.html)
+  for more information.
+- to: This is the date you would like the graph to end - See
+  [http://oss.oetiker.ch/rrdtool/doc/rrdgraph.en.html](http://oss.oetiker.ch/rrdtool/doc/rrdgraph.en.html)
+  for more information.
+- width: The graph width, defaults to 1075.
+- height: The graph height, defaults to 300.
+
+  Example:
+
+```curl
+curl -H 'X-Auth-Token: YOURAPITOKENHERE' https://librenms.org/api/v0/services/localhost/35/graphs/loss
+```
+
+Output:
+
+Output is an image.
+
+
 ### `get_port_graphs`
 
 Get a list of ports for a particular device.