Second Y axis on Stacked Area Chart

Hi,

I'm trying to create a stacked area plot that comprises several different increments that I want to stack on top of each other through time. Easy enough.  However, where I am struggling is that I would like to add a cumulative line plot that sums all of the increments through time on a secondary axis.  Ideally I would not convert the stacked area plot to a bar chart as my data set is large and the granularity of the data gets lost when summed up in a bar chart.  Please see the plot in excel below that I am trying to replicate.  All help is greatly appreciated.

Many thanks in advance,

Chris.