This image shows a Process History Chart that contains both a graph and an events grid. The graph contains a legend and trend lines. Each element (Graph, Events, Trend Lines, and Legend) are described below.
Trend Lines
Trend lines are plotted on the graph and display real-time and historical process data collected on a parameter reference. Trend lines can also be configured to show statistical markers for a trend's minimum value, maximum value, and time-weighted average. You can move a trend line back or forward in time and expand and compress a trend line. To select a trend, click its parameter reference in the Legend. Each graph can contain up to eight trend lines in different colors.
Graph
The graph contains trend lines, the X and Y scales, and the Legend. Trend lines are plotted on the graph and display real-time and historical process data collected on a parameter reference. The Y scale, which runs vertically along the left side of the graph, plots data values. The X scale, which runs horizontally along the bottom of the graph, plots the time scale. Each graph can have up to six Y scales and up to eight trend lines in different colors. The Legend contains information about trends that helps you to read the graph. Select a trend by clicking its parameter reference in the Legend.
If a graph displays more than six trend lines (eight is the maximum), the seventh and eighth trend are displayed on the sixth Y scale. If you use eight trends, it is recommended that you configure items with similar engineering ranges in the last three trends (trends 6,7, and 8) because any scaling will affect all three trends simultaneously. Display string data values in one of the first five trends.
Legend
The Legend shows information about a trend, such as the parameter reference, descriptor, value, units, and timestamp, which help you to read the graph. Additional information can be shown in the Legend, such as statistical values, by adding new columns to it in the Legend Columns dialog.
- The Value column may contain the message Failed or No Data. Failed indicates that the Continuous Historian is unable to retrieve data because an error has been detected. For example, this could happen if the controller loses power. No Data is a generic message, indicating that no data is available.
- You can configure statistics to show in the graph only, in the legend only, or in both the graph and the legend.
Events Grid
The events grid shows continuous events for the parameter references that are trended on the graph. Each event displays horizontally in an event record. You can set event filters to display only those events that you want to see and sort the event columns. An asterisk next to a column heading means that a filter has been set on that column.