Unfortunately Excel does not provide Step charts. So the transition between data points in descending or ascending or it is smoothed and not sharp dropping.
If something you want to do should look like this:
Then have a look at the link below how you can trick Excel into doing it:
http://www.exceldashboardtemplates.com/how-to-easily-create-a-step-chart-in-excel/
It worked fine for me!