You always wanted to know the difference between a combined & dual axis? Here is the main difference you need to know. A combined axis only shows one axis and only allows for one graph type to be shown whilst only utilising one marks card, whereas a dual axis allows for multiple graph types to be shown on a synchronised axis whilst utilising separate marks cards for each graph.
Combined Axis example:
Dual Axis example:
Each type of graph can be used in certain situations depending on what you want to portray to the user of said chart.