Hi Silvio,
create a line chart with two or more series. Then, in the Chart Widget Configuration under "Chart Engine Designer" -> Structure -> Series (Y) select one of the two series by clicking the edit-icon (the one after the up/down-arrows). In the now appearing settings of the series, you should be able to select either "Bar" or "Line" in the "Series item order type". Does that help? :-)
Best
Sebastian