A temperature chart is useful only when its readings describe the same process. Do not combine different products, vehicles or measurement points into one series.
For each record, keep timestamp, route, product group, measurement point, instrument ID and method. Mark changes in load, vehicle, season or sensor on the chart.
Plot readings in time order and state the assessment method beside the chart. If statistical control limits are needed, have a qualified analyst calculate them from adequate data using a suitable method; product specifications and alarm thresholds are not substitutes.
If the chart shows a sustained shift or wider spread, review the relevant records: route, stops, equipment, door openings, calibration and shift notes. Write down the suspected cause and how it will be checked.
A trend chart supports process review; it does not make product acceptance decisions. Handle out-of-specification readings separately under the existing deviation procedure.