try to add horizonal line in chart via vba

If just wanting a horizontal line at the Y value in A1, maybe:
NB: Not tested on a Mac

   ' Retrieve x-values from column Q for the horizontal line
    Set xValuesQ = wsData.Range("Q2:Q" & lastRow).SpecialCells(xlCellTypeConstants)

    ' Add horizontal line series using x-values from column Q and y-value from A1
    With chartObj.Chart.SeriesCollection.NewSeries
        .xValues = xValuesQ
         ReDim xVal(0 To xValuesQ.Count - 1)
         For i = 0 To UBound(xVal): xVal(i) = wsData.Range("A1").value: Next
        .Values = xVal
        .Border.Color = RGB(255, 0, 0) ' Red color for the horizontal line
        .Name = "Horizontal Line"
    End With

    ' Formatting of chart including axis titles, colors, tick marks, etc.