On the marks card you're including City and Continent in the detail, so it looks like you're expecting two polygons; one for the big house looking object you've got and one for your sample city.
However, your sample city is only one piece of data, it doesn't have enough points render a polygon. You would need at least a second lat/long combination for the city to render a line, a third for a triangle etc.
Once you add that, you need to change the order of the 'City' dimension so the sample city shows in the background. Here's an example I created with a couple of extra lines of data in your Sample City;
great, this will work, many thanks! I guess it would work with a dual axis, too.