Improve SEP/SIMPLE/401k post with where() calls

At the right-hand edge of each salary graph in the original version of
this blog post, you would actually be losing money from your business
bank account each year because of Social Security and Medicate tax
payments; so in this improved version of the post, we end each line of
the graph early to reflect the point at which the total burden of
payroll, payroll taxes, and retirement contributions drives the business
income negative.