Area charts (aka area graphs) are charts which encode data using 2D representation. The term is most commonly used to refer to line graphs with the space between the series line and the x-axis filled in, although it can be taken to include filled versions of other chart types such as pie charts and radar charts.
Some data visualization theorists disdain the area graph's fill as a form of chartjunk, with the line graph version held superior. Some studies suggest that the filled area distracts the viewer and reduces the viewer's ability to draw clear inferences from the shape of the line.
Area charts with a soft or gradient fill are nonetheless quite commonly used in financial reporting and performance dashboards, with the idea that they give weight and visual appeal to the data.
Area charts with more than one series are problematic, as they are subject to occlusion where the "front most" data series can partially or even fully obscure the remaining series. The stacked area chart attempts to resolve this by drawing the series one atop the next, but this leads to further difficulties in interpretation because all series except the first do not start from a consistent horizontal base, which distorts their shape and thus hides the true pattern of change for that series. That said, some like the stacked area chart for its ability to show how the parts (say, for regional sales) contribute to the whole (e.g. the total sales figure).