Did Drug Prices Really Decline By a Record Amount in 2018?

Last night President Trump said this:

Already, as a result of my Administration’s efforts, in 2018 drug prices experienced their single largest decline in 46 years.

I’ve been puzzling over this ever since he said it. What does he mean? I assume he means prescription drugs, and most of the fact checkers think his statement is based on a report last October from the Council for Economic Advisers, which contained this chart:

I don’t know if this is correct, but even if it is it only goes back five years. I tried doing a simple look at spending on prescription drugs, but even there I could only find annual data going back to 2000. I gave Trump every chance I could by adjusting for inflation and population growth, but even then I only got this:

Prescription drug spending fell 2.6 percent in 2012, and that’s the biggest drop in the past couple of decades. There’s no data yet for 2018, of course, so it’s not on the chart. It’s also unclear how Trump could have said anything about 2018 since he doesn’t have any data either.

I dunno. I guess he just made it up. But if anyone can clue me in about where he got the idea that drug prices declined more in 2018 than any year since 1972, I’d sure be interested to hear it.