Not over all, however, the 1930's and 1940's has the highest shift in the apex.
Is the count changing for each degree above or below the baseline. That is, if summers are indeed cooling then the number of days at temps above the baseline should be dropping. Indeed they are:
Because there are a different number of over all days in the last years (due to missing records), this was done as a percent of each year's count of records. 5C, 6C and 7C anomalies above the baseline are dropping. Fewer days are in each of those ranges. The drop is steeper with 8C, 10C and 12C above baseline:
But what about the other end, below the baseline, is that changing with time? Doesn't appear to be, -5C, -10C and -15C deviations are flat trends:
So what is happening is since 1920, at least, there are fewer days in the hotter ranges of the year. Not changes in the mid-range temps. This shows a narrowing of the range of swings in temps. And it is not not just the highest of TMax, but all the range of temps of TMax above the average. Summers are not getting as hot today than they did in the 1930s and 1940's.
So over time since the 1920's at least, the bell shape of each year is tending to narrow with fewer days above the baseline, but taller near the baseline. Something to test for.