"I suspect that IRA considers 1999 to be "irrelevant" because a Democrat, Bill Clinton, was in the White House."
No, it is irrelevant because this was pre-wholesale hollowing out of our industrial base due to NAFTA and off-shoring to China. Most of those dirty manufacturing jobs that Liberals hate are the kind that provided health insurance. That is why the trend was coming down from 1999 regardless of GDP growth, etc up to the 2007 crash.
""How many more people are being covered by health insurance than before Obamacare?" BTW, that number is very close to 20M."
Well, let's analyze that. In 2017, total uninsured was 27.4M. In 2013, that number was 42M - so we have a total change of 14.5M. But, over this timeframe employer based coverage (per your prior link) increased by 7M. Also during this time, medicare enrollment increased by 6M. So, absent 'Obamacare' - we would have had a net decrease in uninsured of 13M.
So, that leaves the mystery of where these 8M Obamacare enrollees came from - either we had a population explosion during this time OR the Obamacare enrollees were not really uninsured previously, they just switched to Obamacare from other insurance plans. So, again - how much impact did Obamacare have on the total number of uninsured in the country?