I've been trying a few different things with this, but I can't really think of a workaround on this that does exactly what you want. The problem is there's really no way to get an area chart to start at a place other than 0, which is what this is doing.
However, there are two alternates that might work for you. See the attached workbook. The first is a simple area chart, with a line chart overlaid (so the lines are absolutely clear). You could play around with the coloring to get the best combo for your needs.
The second/third charts are Gantt Bars, which can look like bar charts that start at a value other than 0. To do this, I created a few calculated fields (all of the calculated Measures named "Sales__"). I basically calculated all of these to find the starting values, color criteria, and differences for every month shown. Then in the second sheet, I overlaid a line chart, which is, I suppose, the closest to what you have above that I could get.
I hope this helps a little, though perhaps someone else will have a different solution!
See this thread from the spring: http://community.tableau.com/message/175980?tstart=0. It didn't have different shading depending on which measure was higher, but I think there's enough there to get you started.