A configured pen does not show on the trend.
On the Pens tab, make sure that:
On the Y-Axis tab, check the chart range, as defined by the Minimum / Maximum Value options. If the pen's data is all outside of the specified range, the pen's trend line draws at the top or bottom of the chart, where it might not be noticeable.
Options on the Display tab of the Trend Object Properties dialog box
Options on the Pens tab of the Trend Object Properties dialog box
Options on the Y-Axis tab of the Trend Object Properties dialog box
Keywords: troubleshoot, trend