This article explains how to display retained metrics for the HTTP monitor's Text to look for field.
You can configure the HTTP monitor to retain the results of the Text to Look for field for graphing purposes. However, the option to generate this graph is greyed out when viewing the service metrics because the values are stored as text strings, which cannot be plotted on a graph like integers or other numeric values.
The solution for viewing the retained results for this monitor is to generate a Service Monitor Metrics report, which provides a list of the monitor's historical results.