There's is an interactive chart that's easier to understand
That's what El Niño is about: instead of declining after a peak in June, temperature plateaus and then rise again later in the year.
So the fact that the Pacific is much hotter than it was in 2015 is particularly scary, because it leads us to believe that it will also be this hotter than 2015 in November.
The graph has a key on the right hand side that clearly labels each colour of line, and the horizontal axis is scaled in months of each year. Scrolling down gets you notes and links to data sources.
In answer to another poster in this thread, the dataset only reaches back to 1983, I'm assuming because that is when they started monitoring these temperatures?
The baseline of 0.0 represents the average of all years. Anything above / below the baseline is a (standard) deviation from the average. The blue lines are the individual years since 1991 [1] while the red line is the year 2026.
If a line is above the baseline, then the sea-surface temperature was hotter on that day than average. If below, it was cooler than average.
The year 2026 is an outlier, dwarfing all the others starting around June / July. The Nino 3.4 sea-surface temperature is significantly hotter than any previous year during that time. New record, I guess?
[1]: I'm confused about the two date ranges given: 1982-2026 and 1991-2020. I'm assuming this graph is based on measurements from 1982-2026 to calculate the average, but the lines shown are only from 1991-2020, for some statistical reason I don't understand.
standard deviation is a measure that informs about the distribution. A high standard deviation means a "wide bell curve". A low standard deviation means that all values are closely clustered around the middle of the curve.
So if your value is 2 x standard deviation (for example) that means it is a relatively rare outlier, since 2 x standard deviation covers 95% of the bell curve. In particle physics I believe they require 5 standard deviations to confirm an observation.