To reverse the legend for a graph,
Click Settings on the right side of the Visuals menu.
The Settings modal window appears.
Select Reverse Legend Order.
Click Refresh Visual.
Note that the legend is initially appears in ascending alphabetical order, and then reverses to descending alphabetical order.
For graphs that map a continuum of values, such as choropleth maps, the legend shows a sliding scale of color and values at the minimum, median, and maximum. The reverse legend option simply changes the orientation of the legend from maximum value on top, to minimum value on top.