What I suggest doing is creating a graph on the form where it will reside. Use the wizard to feed it the data that you're interested in, including the information for both the bar and the line. Then, after the graph is displayed, double click on it; this will allow you to edit the graph's data as well as it's layout. To modify one of the series, click on the series in question, and then right click and select 'Chart Type'. Change the type from bar to line, and it should do the trick. The important part here is to just select a single series when you're changing the chart type; it's counterintuitive, but that's how MS set it up.
