The 2018 figure is the average of seasonally-adjusted figures for June and July, 2018 (which is a good projection for the annual average). Previous years are annual averages.
We also have a similar graph for methane (CH4),
and a combined graph with both CO2 and CH4.
Direct measurements of CO2 at Mauna Loa Observatory, in Hawaii began in March, 1958. This is the annual average data we’ve graphed:
ftp://aftp.cmdl.noaa.gov/products/trends/co2/co2_annmean_mlo.txt (1959–2017)
For monthly data see:
ftp://aftp.cmdl.noaa.gov/products/trends/co2/co2_mm_mlo.txt.
1850-1958 data is from http://data.giss.nasa.gov/modelforce/ghgases/Fig1A.ext.txt (ice cores).
1800-1850 CO2 data is from http://www1.ncdc.noaa.gov/pub/data/paleo/icecore/antarctica/law/law_co2.txt (Law Dome ice cores, 75 year smoothed).
SeaLevel.info