To stack an area chart simple create a new chart or edit an existing one .
Make sure you have 2 or more metrics being plotted similar to the example below.
From there you'll want to navigate to the Options tab in the top right.
From there select the Area chart type and below that there is a dropdown for Stack Type, select Normal.
Note: you could also stack the chart by percent instead of actuals as well.
Click save and you'll be on your way! Let us know if you have any questions.