Graphite Derivative shows no data - graphite

Question

Using graphite/Grafana to record the sizes of all collections in a mongodb instance. I wrote a simple (WIP) python script to do so:
#!/usr/bin/python
from pymongo import MongoClient
import socket
import time
statsd_ip = '127.0.0.1'
statsd_port = 8125
# create a udp socket
sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
client = MongoClient(host='12.34.56.78', port=12345)
db = client.my_DB
# get collection list each runtime
collections = db.collection_names()
sizes = {}
# main
while (1):
# get collection size per name
for collection in collections:
sizes[collection] = db.command('collstats', collection)['size']
# write to statsd
for size in sizes:
MESSAGE = "collection_%s:%d|c" % (size, sizes[size])
sock.sendto(MESSAGE, (statsd_ip, statsd_port))
time.sleep(60)
This properly shows all of my collection sizes in grafana. However, I want to get a rate of change on these sizes, so I build the following graphite query in grafana:
derivative(statsd.myHost.collection_myCollection)
And the graph shows up totally blank. Any ideas?
FOLLOW-UP: When selecting a time range greater than 24h, all data similarly disappears from the graph. Can't for the life of me figure out that one.

Answer 1

Update: This was due to the fact that my collectd was configured to send samples every second. The statsd plugin for collectd, however, was receiving data every 60 seconds, so I ended up with None for most data points.
I discovered this by checking the raw data in Graphite by appending &format=raw to the end of a graphite-api query in a browser, which gives you the value of each data point as a comma-separated list.
The temporary fix for this was to surround the graphite query with keepLastValue(60). This however creates a stair-step graph, as the value for each None (60 values) becomes the last valid value within 60 steps. Graphing a derivative of this then becomes a widely spaced sawtooth graph.
In order to fix this, I will probably go on to fix the flush interval on collectd or switch to a standalone statsd instance and configure as necessary from there.