Metric is an aspect of the object that can be measured and plotted on the graph. For example, you can measure temperature in the room, voltage in the electric circuit, and amount of free space on disk. Each property has a name usually referred as a metric (or measurement) name, e.g. ‘temperature’, ‘voltage’, ‘hddfree’. Metric names should be unique within object. For example, if you want to measure amount of carbon monoxide in the room using two sensors you should use two different measurement names for both sensors. Otherwise you won’t be able to distinguish between measurements of both sensors: