The data comes from the US HCN, the United States Historical Climatology Network, and the graph seems to show that the number of 90-degree days is been trending down over time.
Here's the question: is the number of 90-degree days per year a good proxy for the average US temperature during the summer? I'm not sure it is. The next chart tells the tale:
The chart comes from NOAA, and you can play with the time series. I've chosen to chart the average US temperature of the "lower 48" for the summer months of June, July and August.
The 90-degree day chart doesn't look like a good proxy at all: its trend line is sloping down, but the actual average summer temperature is sloping up.