When rendering line charts, tableau connects each mark by a straight line, in effect, doing a linear interpolation between the data points as if the measure changes continuously between each pair of data points.
There are problem domains where this is misleading and the best way to draw a line chart would be to keep the previous value constant until the next data point, and then instantly step up or down to the new value. I think matlab calls them stair plots.
Consider plotting inventory levels, account balances, queue lengths, voltages in a digital circuit, usually with time as the x-axis. In these cases, the current Tableau approach leads to misleading visualizations -- suggesting for instance that the inventory or bank account value changes continuously over time, rather than at discrete instants (say when there is delivery to a warehouse, or a charge to a credit card). This problem is especially aggravating when there are long periods without any change (and thus no data rows), so that Tableau interpolates across a long time period, suggesting an inventory or account level that is not accurate for the days without any transactions. There are workarounds but they are tedious.
The simplest solution I can see is to add an option to the mark shelf when the mark type is line -- to allow the user to select whether to draw a straight line between pairs of marks or to change value in discrete steps. You'd probably need to do the same for area charts.
--> Community Manager update [Ciara Brennan]
Feature released in Tableau 2018.1
Step and Jump lines | Feature 80974