I REALLY appreciate the help this forum provides!
I saw some similar topics to this one, but they didn't seem to address what I think might be an initialization issue. If this is a duplicate then I apologize!
Using Vision 8.1.26, running on Windows 10.
I was puzzled to notice that setpoints were not plotting on an Easy Chart when I had seen them plot on an almost identical system at another site. It seemed odd because if I included the tags on the trend and then used the X-Trace, the correct values would be displayed, but no trend line would be plotted.
It took me a while to realize that it had to do with the fact that the Setpoints were the same value that was loaded with the initial PLC program and have never changed. I tested that theory by tweaking one of the setpoints, and sure enough it would plot from the time I changed it onwards.
Is there some kind of initialization that needs to happen with these values that rarely (or never) change?
Or maybe I just have something set up incorrectly with the Easy Chart?
Or do I need to set these tags up to always be read at a particular frequency (the are currently set to the 'Tag Group' sample mode with the Max Time Between Samples = 0)?
Or...something else?
Many thanks in advance!
Yeah, this is bad. If you don't set a max time lower than your most-zoomed-in chart span, you can't be sure the pen will show up at all. (If no point in the span, there won't be anything to plot.)
Unfortunately, zero is the default, as that allows the best space efficiency. 
The tag historian doesn't extrapolate or interpolate so problems such as you are describing show up in various situations. When I started I expected it to look back to find the most recent reading at or before the start period and interpolate to give what might be considered a reasonable value for the start of the time period. It doesn't! And you can have similar problems at the end of the period. This limits the default historian usefulness quite a bit. As you now know, you can "fix" this to some extent by configuring an appropriate max time between samples. So, if you're interested in daily charts a 4-hour max might be appropriate, etc.
If you need to calculate daily totals, 15 minute utility meter readings, parts produced each hour, then you need to create your own periodic logger. The Project → Gateway Events → Scheduled can be a good start. You'll need to create your own table for this.
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Thanks for your responses! What you are both saying makes sense, but this still confuses me:
These are the configurations from two different sites for the same tag. The one on the left doesn't plot anything, but the one on the right does and always has.
That's what lead me to think that I must have something else set up wrong. Any thoughts about why one works and the other doesn't? They both essentially have a constant value in them, although the value at one site is slightly different from the other.
I will say that the one that works uses a little older version of Ignition (8.1.22 instead of 8.1.26), and they use different CompactLogix PLCs. But I can't think of any reason that either one of those differences would matter.
Are both tags the same data type? Since both are set to auto on deadband style if one is an integer and the other is float or double, that might explain it.
Different Data Types are a good thought!
They are both 'REAL' data types at the lowest level. However, they are both contained within a UDT structure in the PLC. The structures in the PLCs are identical. But that does make me wonder if it has something to do with how the UDT's might be handled between the different versions of Studio5000 and Ignition?
The older one uses version 30.02.00, while the newer one (the one that doesn't plot) uses version 34.01.00. I seem to vaguely remember that there was something that changed regarding UDT interaction at Studio5000 version 34.
The only way I can think to test that theory would be to wipe a test PLC and install the different versions into it and see how they both respond. Unfortunately, I'm under a lot of pressure to get other things done, so I won't be able to commit the time that would be required - at least not for a while.
