I think there is an undesirable behavior in the chart components when displaying the tag history of a tag with “Value Mode” set to “Analog”.
In this condition the tag history will use the compression method that is well explained in the manual, so the configured deadband is used to form a corridor along the trajectory of the value. The problem is that the chart components at the current time display the last stored value in the historian database, not the projected value based on the tag trajectory. This could lead to gigantic errors on the chart when in realtime mode.
Just few weeks ago I was fooled by this behavior: I was following the internal temperature of a reactor with a realtime chart with time span of 10 minutes that was displaying a constant 70°C, while the measured internal temperature was increasing linearly and was around 80°C! At the first glance, I thought that the historian was not working properly…
I hope my explanation is sufficiently clear.
At the moment, I set all “Value Mode” of the historian to “Discrete”, so the last stored value is near the current value (the deadband is on the last stored value, not on the trajectory). The downside of this solution is that now the easy chart displays the analog value using steps and there is no way to change the way a line is rendered, because the choice is automatic, based on the “Value Mode” (while it is possible to set the behavior of the chart component, so I am thinking about a switch from the easy chart to the chart component).
I think you should consider the idea of adding a new row with the current datetime and actual tag value (or the projected value using the last two stored values) to the dataset returned by the function system.tag.queryTagHistory in case of “Value Mode” of type “Analog”.
Thanks.