A demonstration of the filledAccumulative Line chart setting
Documentation about the Line chart setting filledAccumulative. By using this setting you can effectively stack your filled lines on top of each other. This may be useful if you're trying to see the summed values of your datasets.
This page shows how the setting filledAccumulative affects your charts. The chart on the left has it enabled (the default), so the lines in effect sit on top of each other. The chart on the right has the setting set to false so the points are drawn "as-is", ie not accumulatively. The first chart could be more useful if you wish to see accumulated values, whereas the chart on the right might be better if you want to see individual values.
Keep in mind that if you choose to use a non-accumulative chart the colors may be better as semi-transparent ones, otherwise parts or all of some lines may get hidden by others.