You should be familiar with these definitions before reading the URL API
A value for a given time.
A metric is a named set of datapoints ranging over time. You determine the name for the data when you send messages to port 2003. An example would be system load on a server called apache02 in datacenter MetroEast:
MetroEast.servers.apache02.system.loadAvg
Time is represented on the x-axis.
The smallest time that Graphite can currently save is one second, however, many metrics are recorded once a minute. Others may only be kept hourly or daily. You pick the length (of time) of one datapoint by configuring storage-schemas.conf before sending the first datapoint.
If you send more frequently than defined in storage-schemas.conf, by default Graphite drops the old value and replaces it with the last one it receives. You can configure a hit-count function, to add all values, instead of replacing, or have your data collection agent aggregate and send only one value.
When creating a graph, a dot is place above each time on the x-axis, at the value on the y-axis, and a line is drawn connecting them. If no data was sent, the line will not be drawn and you will see a gap. When using any function that combines many lines (metrics) into one, the lowest resolution (the longest time period per datapoint) is used.