PI Vision guide · Daily trend comparison
How to Compare Today with Yesterday in PI Vision
Today’s process trend looks different—but when did it diverge from yesterday? Overlay the same measurement at the same time of day with Period Comparison Chart+, then inspect the interval that needs attention.
Keep both curves visible: compare 05:00 today with 05:00 yesterday, without switching dates or relying on screenshots.
Before you begin
This guide uses Period Comparison Chart+, a Vision Library+ extension for AVEVA PI Vision. You need the symbol available in your installation, permission to edit the display, and one numeric PI Point or AF Attribute with readable history for both days.
Choose a measurement such as temperature, pressure or power. Keep the asset and units the same across both periods. This is a calendar-day trend comparison, not a shift comparison or a calculation of daily production totals.
1. Add the measurement and anchor it to today
Open the display in edit mode, select Period Comparison Chart+ from the symbol gallery, and add your measurement. Right-click the symbol to open its configuration pane.
Set the time range driving the symbol to today. If you use the Vision Library+ Time Bar, enter t for the start and * for the end: midnight today to now. Check the displayed dates. A custom or locked symbol time range can override the display range; see the Time Bar documentation if they do not follow the display.
The anchor is the period the comparison starts from. If it still points to an old date, the chart will compare historical days—not today and yesterday.
2. Create two calendar-day layers
In Layers, apply the settings below. A layer is one time period for the same measurement. Calendar mode aligns the days to their boundaries instead of comparing two rolling windows.
| Setting | Value | Purpose |
|---|---|---|
| Mode | Dynamic | Generate the comparison from a repeatable rule. |
| Period Type | Calendar | Align the periods to calendar boundaries. |
| Duration | 1 day | Use a daily comparison axis. |
| Count | 2 | Show the anchor day and one earlier day. |
| Direction | Past | Compare with an earlier period. |
| Shift By | Calendar / 1 day | Place the earlier layer one calendar day back. |
For one measurement, set Distinguish Layers By to Color so the two days are easy to identify. In the data source’s layer settings, leave Anchor Only disabled; otherwise that measurement will not repeat on yesterday’s layer. Keep extra pinned periods out of this first comparison.
3. Show the trend shape
In Data Retrieval, use Default / Plot. In the data source settings, select Line as the series type. Under Scales, use Time Scale → Range for a continuous comparison and a shared value scale for this single measurement.
Plot is the starting point for finding where the curves separate. Use Recorded when you need exact archived events, or Interpolated when the task requires equally spaced samples. Those modes change what each point represents; do not switch to Summary just to make a trend look smoother.
4. Compare the same time of day
Identify today and yesterday in the legend, then hover at an elapsed time that exists in both periods. Move along the curves to find where their behavior starts to differ. Double-click the chart for fullscreen when you need more room to inspect the interval.
In the product example below, the tooltip at 05:00 shows today at about 4.82 and the previous day, May 27, at about 30.34. That identifies a substantial difference at the same daily position. It does not tell you why the measurement changed.
Next, examine the absolute time window around the divergence in your normal PI Vision trend, alongside the signals that could explain it—for example, a setpoint, operating state or throughput. If the two days ran different products or loads, that context may explain the difference. The overlay gives you a narrower interval to investigate, not an automatic diagnosis.
Check the result, then save the display
You should see the same measurement on two dated layers, a common daily time axis and readable values at the same time of day. Confirm that the anchor date is today and the previous layer is yesterday, then save the display for the next daily review.
If the comparison looks wrong
- Yesterday is missing: check source history and access, layer visibility, the one-day offset, and Anchor Only.
- The chart shows the wrong dates: inspect the display and symbol time ranges, especially a custom or locked Time Bar range.
- The view is crowded: return to one measurement and two layers; remove unnecessary pinned references before adding more context.
