From a post by Greg Mankiw (The One Percent, Updated):
Over the last sixteen years, the share of pre-tax income earned by the top 1% has ranged between 17% and 23%. I calculate an average of 20% over that time frame, precisely where it is today. Read Mankiw's post for more details.
A peculiar note: I suspect if you were to show this data from 1975 forward without the title and the y-axis label, I suspect many people would think this was a graph about global warming.