If you are looking for ideas, have a look at Matt Crowther's blog
Alternatively, search the web for infographics as there are plenty of different charts and graphs that may inspire you and impress your service managers.
As an idea, how about a histogram:
- calculate the number of consecutive years for each customer
- build a bar chart with the count of customers per number of years in service.
You will know where most of your customers fit in.
Another idea is to divide all of your customers into a number of buckets, based on their tenure, and examine the top bucket - what characteristics are different for most of those customers with the longest tenure?