Function trigger for trace offset?

Function GetTraceOffset(graphNameStr, traceIndex, xOffset, yOffset)
    String graphNameStr
    Variable traceIndex     // 0 for first trace, 1 for second, . . .
    Variable& xOffset       // Output
    Variable& yOffset       // Output
   
    xOffset = NaN
    yOffset = NaN
   
    String info = TraceInfo(graphNameStr, "", traceIndex)
    if (strlen(info) == 0)
        return -1
    endif
   
    // Strip out everything before "offset(x)" - (would not be necessary if I knew what I was doing with regular expressions)
    Variable pos = strsearch(info, "offset(x)", 0)
    Variable len = strlen(info)
    info = info[pos,len]
   
    // Strip out everything after next semicolon - (would not be necessary if I knew what I was doing with regular expressions)
    pos = strsearch(info, ";", 0)
    info = info[0, pos]

    // Extract x and y offsets as strings
    String regExp = ".*offset\\(x\\)={([^,]*),([^\\}]*)"
    String xOffsetStr, yOffsetStr
    SplitString /E=regExp info, xOffsetStr, yOffsetStr

    // Convert to numbers and return via pass-by-reference parameters
    xOffset = str2num(xOffsetStr)
    yOffset = str2num(yOffsetStr)
   
    return 0
End

Function MyWinHook(s)
    STRUCT WMWinHookStruct &s

    Variable xOffset, yOffset

    Variable rval= 0
    switch(s.eventCode)
        case 4:         // Mouse down
            GetTraceOffset(s.winName, 0, xOffset, yOffset)      // 0 means first trace
            // Printf "xOffset=%g, yOffset=%g\r", xOffset, yOffset
            Variable /G root:gLastXOffset = xOffset
            Variable /G root:gLastYOffset = yOffset
            break
        case 5:         // Mouse up
            GetTraceOffset(s.winName, 0, xOffset, yOffset)      // 0 means first trace
            // Printf "xOffset=%g, yOffset=%g\r", xOffset, yOffset
            NVAR lastXOffset = root:gLastXOffset
            if (xOffset != lastXOffset)
                Printf "New X offset = %d\r", xOffset
            endif
            NVAR lastYOffset = root:gLastYOffset
            if (yOffset != lastYOffset)
                Printf "New Y offset = %d\r", yOffset
            endif
            break
    endswitch

    return rval
End

Function DemoSetCursorHook()
    Make/O jack=x
    Display jack
    SetWindow kwTopWin,hook(testhook)= MyWinHook
End

DisplayHelpTopic "Window Hook Functions"

DisplayHelpTopic "Background Tasks"