I just figured this out on my own. As an FYI to anyone else, here was my
solution:
1. Plot the number of cats on the primary axis
2. Plot the number of neutered cats on the secondary axis
3. Format the "neutered cats" series to enlarge the gap width

"CParker" wrote:

Sample scenario: Let's say in each year, I have this many cats:
2010: 6
2011: 8
2012: 20

Out of those cats, let's say that the following number have been neutered:
2010: 1
2011: 4
2012: 18

Is there a way for me to create a form of bar chart that shows the total
number of cats per year as a "background" bar, with the total number of
neutered cats in each year as a skinnier "foreground" bar that is a different
color and overlays the background bar?