I just asked a very similar question, but didn't get any answer.
However, I did figure out a way to get it to do what I wanted.

1.  Start with a plain stacked bar graph and chart everything you want
in that format.  If you choose 2 columns for your X-labels, Excel is
smart and will give you 2 levels of X-labels and will space your chart
columns according to the row spaces in your data layout.

2.  Put your line data next to your stacked bar data, but don't graph
it.

3.  Go to the chart and Add a new series under source data to the
chart.

4.  After adding this new series, click on it, change the axis to the
2ndary axis.  And choose change the chart type to "line."

5.  If you had spaces in your data table, put =N/A() function in the
spaces if you want to connect the data points in your line.  This will
but #N/A in your table (which you can make white if you don't want to
see it), but it is the only way to get Excel to continue the lines from
point to point.

Hope that made some sense... good luck!