Add hover over tooltip for axis label

[neka-io]

Earnings - Income and earnings are often confused. In reality, earnings are just one kind of income. Earnings, primarily wages and salary from a job, are usually a big source of income. Other sources of income include Social Security payments, pensions, child support, public assistance, annuities, money derived from rental properties, interest and dividends.
citation (http://blogs.census.gov/2010/09/23/income-vs-earnings/)

Percentile - a measure used in statistics indicating the value below which a given percentage of observations in a group of observations fall. For example, the 20th percentile is the value (or score) below which 20 percent of the observations may be found.
citation (https://en.wikipedia.org/wiki/Percentile)