If you created a graph on a Microstrategy dashboard (document) based on a dataset, and you need to sort the graph in descending order, you need to first switch the graph to grid view.
In my case I had a horizontal bar chart, with my attribute on the y axis and metric on my x axis. I had created the graph on my document based on an existing dataset. To sort my graph in descending order, I had to switch my graph to grid view (right click on graph, click "View Mode", then choose "Grid View"). Then go into the grid edit mode by clicking the square in the upper left corner. On the menu bar, click "Data", then click "Advanced Sorting", and in the "Rows" tab, choose the attribute/metric you want to sort by and select Ascending or Descending order. Switch you grid back to "Graph View".
Below is my graph after I sorted it.