Area charts are valid only if the area is meaningful. There certainly are cases where it can make sense to use a visualization based on filled, stacked areas. Look at, for example, Weatherspark.com&#x27;s colortemp visualization for historical weather trends, which shows &#x27;percentage of time when temperature is in a particular band&#x27; over a time axis. That&#x27;s a good visualization, because the quantity makes physical sense - integrating &#x27;time in band per day&#x27; over &#x27;days&#x27; gives you overall &#x27;time in band&#x27;. Integrating &#x27;market share&#x27; over &#x27;months&#x27; gives you &#x27;market share months&#x27;, which is a meaningless value.