NCM plays each Stockfish dev build 20,000 times against Stockfish 7. This yields an approximate Elo difference and establishes confidence in the strength of the dev builds. As of time of writing, one can see from the plot that Stockfish's strength hasn't really budged for a month: version 20190324-1637 had a performance of +217.95 +/- 3.73 elo relative to Stockfish 7, while the latest version 20190420-0634 is +221.52 +/- 3.79 elo. The gap is within the error bars.
(It's possible that this is sort of the expected performance increase in a month's time, but the latest version's results appear to be anomalously high. For example the next-latest version, 20190419-1533, is +218.61 +/- 3.77 and almost indistinguishable from the 20190324-1637 version.)
The weird thing is, for each version of Stockfish to be promoted to the "main version", it has to beat the previous version. All these new versions of Stockfish (that aren't non-functional changes) have to do this. If we look at the catalogue of Stockfish patches, we see patches such as:
Author: protonspring Date: Tue Apr 16 23:10:53 2019 +0200 Timestamp: 1555449053
Move pawnsOnSquares to Position (#2100)
We can remove the values in Pawns if we just use the piece arrays in Position. This reduces the size of a pawn entry. This simplification passed individually, and in concert with ps_passedcount100 (removes passedCount storage in pawns.).
STC LLR: 2.95 (-2.94,2.94) [-3.00,1.00] Total: 19957 W: 4529 L: 4404 D: 11024 Elo +2.18 http://tests.stockfishchess.org/tests/view/5cb3c2d00ebc5925cf016f0d
Combo STC LLR: 2.95 (-2.94,2.94) [-3.00,1.00] Total: 17368 W: 3925 L: 3795 D: 9648 Elo +2.60 http://tests.stockfishchess.org/tests/view/5cb3d3510ebc5925cf01709a
In other words this version of Stockfish defeated the previous one by ~2 elo. Why isn't this ~2 elo showing up in the NCM data? I know elo isn't transitive (i.e. if engine A beats engine B by 50 elo and engine B beats engine C by 50 elo, it doesn't mean engine A beats engine C by 100 elo), but this is just one patch, and there were plenty of elo-gaining patches that passed Fishtest. For NCM to show no progress for months, in spite of all these patches, doesn't make sense to me.