gistfile1.py

import requests

render_url = "http://example.com/render/"
target = 'your.metric.name.here'
from = '00:00_20140101'
until = '23:59_20140520'

# First omit the format so the request will render as PNG (default)
post_dict = {'from': from, 'until': until, 'target': target}
r = requests.post(render_url, data=post_dict)
# Now request JSON format. No matter what we request now it will come back with the previous response,
# for every method other than GET (assuming the cache is cold).
post_dict = {'from': from, 'until': until, 'target': target, 'format': 'json'}
s = requests.post(render_url, data=post_dict)
# Check what the content-type was. For buggy installs this should be image/png.
print s.headers['content-type']