FAQ 009: Why does my Line Chart only show data for the first week of the month, and then a flat-line (or no data) for the rest of the month?

Many historical data sources, such as process historians, by default limit the amount of data that can be returned in a query.  This is done as a “safety net”, due to the potentially huge amount of data that could be returned from a long-running query.  Certain historians default to a max number of rows (unless the user specifies otherwise), and many of the Dream Report communication drivers also have a default maximum row count (per tag queried) such as 10,000 or 100,000 values.  If, for example, a tag in the historian was being logged on change or every few seconds, the max number of rows would be reached within the first few days of a query over a month, and the Chart (or Table) would only be able to display the actual data returned from the historian.  To avoid this situation, either take advantage of the historian’s advanced retrieval or sampling modes exposed through the Dream Report communication drivers to increase the data retrieval interval or increase the max number of rows setting.