Axes allow us to separate metrics into different scales (to ensure visibility of metrics across different ranges of values). In this example, let us add Total Cost and CPI as the metrics in a combo chart, with the dimension as Audience Type from Facebook.
As you notice, Cost and CPI are now on the same scale, and we would need to change CPI to a different scale to make it visible (values of cost for each dimension value are far greater than the values of CPI). To do this, click on 'Style' -> 'Axes' and pull the CPI into the new axis box. There's no limit on the number of axes you can have in a chart.
Continuing on the same example, let us say we want to visualize total cost as an area instead of as columns. To do this, click on 'Style', hover on the chosen metric, and pick the style that you want to show it in. A chart can have different metrics in all the four available styles.
On the same example, let's say we want to view CPI in red color instead of blue. To make this change, click on 'Style', hover on the chosen metric and pick the color that you want to show it in. You can choose different metrics to have the same color too, if you so desire.
Labels are useful to see the values of the chosen metrics for each dimension value in a chart. To enable them, click on 'Style', and check the box next to 'Show Labels'.