An area chart is a line chart with the space below the line filled in. Set the mark type to Area on a view with a date on Columns and a measure on Rows.
With a dimension on Color, Tableau stacks the areas: each band sits on top of the one below, and the top edge is the total. That's the strength of a stacked area (the total and its parts in one view) and its weakness: only the bottom band has a flat baseline, so the bands above it are hard to compare.
Two fixes are worth knowing. Remove the color to see the total trend on its own. Or turn the measure into a Percent of Total that runs across the colors (Compute Using > Category), so the stack always reaches 100% and you see how the mix changes.
Continuous dates (green) draw one smooth area. Discrete dates (blue) break the area at each year, like a line.