The x axis depends on the mode. It displays period if you are in summary view. In variance mode it displays the selected dimension.
The y axis displays the amount. this example, the user is in summary view. They want an 2017 in 2016. The benefit of selecting a waterfall chart over a bar or column chart is that you get to see each piece of the puzzle. For example, hovering on the bar for December shows you there was a 77% decline in revenue just for that month.
Accessing the waterfall chart
Choose Chart and select Bar
What is displayed
The same information in a column chart.