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When you create a graph as part of a trend report (see Reports), you specify either a relative or absolute time period for the graph.
If the database has been configured to store a sufficient number of raw samples to cover the period specified (default is 120, enough to cover five days), the graph will be based on these actual datapoints.
If there is not a sufficient number of samples, the graphing process will obtain all of the values needed to plot the graph from the calculated averages. (Whether a graphline plots average, maximum, or minimum values is a user-defined option, specified when you set up the trend query. See the section Trend Reports elsewhere in this chapter.)
This shift from actual to averaged values can produce unexpected results.
For example, if you define a graph on Friday to show values for the week just ending, the graph line will be plotted from actual, gathered values. (Assume the graph uses absolute dates for the previous Monday through the current Friday, and all database tables are default sizes.)
But if you save the trend report and recall it the following Monday, the entire graph line will be plotted from the daily average, not hourly, values.
Instead of hourly fluctuations, there will be a single flat line for each 24-hour period on the graph’s X-axis. (Or, if one of those daily averages had been marked invalid because of insufficient values to calculate an average, then the graph would contain a gap for that day.)
By default, the database stores 120 hourly values, enough to plot a graph covering five days. On Friday, when you created the graph, there were enough hourly values to cover the specified time, beginning on Monday.
But by the following Monday, the 120 hourly values were not sufficient to construct a graph dating back seven days, to the previous Monday. There were only enough values to plot back to the previous Thursday. Because there were not enough hourly values to cover the specified, absolute time period, the graphing utility used daily average values to plot the entire graphline (not just the portion for which it lacked hourly data).
Many trend reports plot multiple graphlines on a single chart. Each plot line is calculated independently of the others. It is possible to have one plotline based on average values in the midst of other plot lines that use hourly values.
If you had changed the size of the daily table from its default of 30 to less than seven, there would not be enough daily values to plot the graph either, so the utility would use the weekly average to plot the graph. You would then see a single flat line with no fluctuations, either hourly or daily.
The legends below the graph will indicate the kind of data used for each graph line, in square brackets: H for hourly, D for daily average, W for weekly average, and M for monthly average. If the time between datapoints is less than hourly, the actual interval, in seconds, will be displayed.
To increase or decrease granularity of the display, click on the graph to select it, and use the magnifying glass buttons which will be displayed.
NOTE: When you save a trend report, regardless of whether the time period is absolute or relative, you are saving the query criteria, not the data used to plot the graphs. The graphing utility will retrieve data from the database each time you display the report. If you want to preserve a specific graphline, use the e-mail feature to send the report as a Portable Network Graphic (.png) image.
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