Create the chart with both series... using one of the desired chart types.
Select one of the series (you should see selection handles around each of
the bars or on all points of the line) and change the chart type.  With a
single series selected, it will apply only to that series, creating a combo
chart of more than one type.
With a bar chart and line chart, it will create a second x-axis for the line
series, using the horizontal one for the bars... which is not exactally
superimposed.  I think it would be more what you were looking for if you used
a column/line chart combination.
Good Luck!