The YAXIS section in a TrendSpec definition has to contain one YAXIS Mandatory Value. The string that is specified for this value has to be identical to the string specified for the same mandatory value in the corresponding TRACE section.
A TrendSpec definition has to include one TRACE section and corresponding YAXIS section per trace. Therefore, a Trend that has multiple traces has to have multiple TRACE and corresponding YAXIS sections (see Multiple Trace Ad Hoc Trends).