A range must be specified for Historical graphs, it determines the time frame that will be represented on the graph. (e.g. Today, This week, Last Month, This Year, ...) You also specify the scope of each interval represented on the graph, e.g. 1 minute, 15 minutes, 1 hour, 1 day. Those combined options allow you to get a graph that is easily readable at all times.

You can also specify what we call a comparison period. A comparison is a pre-defined time interval during the day. e.g. On weekdays from 4pm to 6pm. This option, combined with the one above, allow you to target a specific time frame within the selected range.