Why can't I see the change value or the trend line?
Change and trend need at least two measurement days made with the same set of prompts and engines. They don't appear for a while after your first scan or after your prompt set changes.
After your first scan, the chart has a single measurement day, and the card notes that a trend line needs at least two scans. The line appears after the second scan day.
If you've added or archived prompts, or turned an engine on or off, the new measurement isn't compared with the previous one and no change is calculated. The bell on the Overview shows a “Coverage differs from the previous measurement” note. Once two measurements have been made with the new set, the change is calculated again.
The same applies when an engine couldn't be measured in a round: that day doesn't have the same set as the previous day, so no change is calculated until the next complete measurement.
If you selected only a day or two as a custom range, the period may contain just one measurement day. Choose a longer period.
The “W” (weekly) button at the top right of the chart becomes available when the selected period has at least 14 measurement days. Since scans run every two days, this requires at least Last 30 days to be selected.