You can mix almost all chart types together in a mixed graph. Some exceptions are going to be maps, bubble pie and boxplot. These all take a more intricate approach to loading the data. If you cant use two graphs in a graphset you can always overlay one graph on top of another. Although this is not ideal, this is essentially creating your own graph type. The easiest way to test out if the chart types work together is to click edit and change the 'type' attribute and see what the results are!
Refer to the Mixed Charts Tutorial for more information.