"The rich get richer"? - is this a Law a Nature?

... alternative title:

Yes! - Let's TALK about the redistribution of wealth!

Below is a graph depicting another ratio:

Household income:

(Upper 5%)/median

I went to the trouble of drawing a line through the points - it's pretty darn linear!

If this were hard science one would be tempted to say, "Hey, we've discovered a Law of Nature!"
I doubt Galileo's data for his Law of Falling Bodies fit a curve quite so well.

In 1975 - the earliest year for which I have complete data, the top 5% of households earned about 2.7x the median household.
In 2014 - the latest year for which I have complete data, the top 5% of households earned just over 3.8x the median household.

Note:
I'm talking about the upper 5% of households, not the infamous 1%.
These aren't the super-rich, just the "pretty-darn-well-off" folks.
The median? - Well, those are the folks smack-dab in the middle - the truly MIDDLE class.

Every year, year after year...
... the top 5% of households gain about 2.8% on the rest of us!
What's going on here?
IS this a Law of Nature?