Vertical baselines duplicating per series

[htimsluap]

I am trying to add vertical baselines to a chart with multiple series - it displays correctly when there is only one series but as soon as I add another the baselines are duplicated.

i.e. in the attached screenshot I want the baselines in the chart on the right to be shown as in the chart on the left

I created the baseline by adding a data item [Baseline] to the chart query with the following expression:
if ([Order date] = _last_of_month(?date?) or [Order date] = _last_of_month(_add_months(?date?, -1)))
then (1) else (0)
then adding a member value baseline to the chart with report expression [Chart Query].[Baseline] = 1