The animation below shows a single sigmoid curve using a logistic function. In the text below, the value of t is shown, a number from -6 to 6 representing how far through the curve the animation is. This value is what is used to determine the coordinates for intermediary values in Tableau.
To build many visualizations using the sigmoid curve (such as the Sankey Diagram), the X or Y positions are held constant for multiple paths or polygons in the visualization. For example, rankings can be shown on the Y axis for two years, with smooth curves showing the change in rank. Drag the control points to the right to see how changes in one dimension effect the curves