The relative scaling of the CO2 and CH4 axes was chosen so that the relative visual magnitude of changes in the two traces would very roughly approximate the relative warming effect of changes in the levels of the two gases. Additional CH4 is variously estimated to have between 20x and 80x the warming effect of the same amount of additional CO2; we used 25:1 so that the grid lines would line up nicely in the graph (and it is also very close to the 26.5:1 figure from the first two rows of AR6 Table 8.A.1, p.731 [alt]), but 40:1 or 45:1 would have been more accurate. However, the absolute height of the CH4 trace in this graph, relative to the bottom of the graph, still greatly exaggerates its total warming affect, compared to CO2. ↑
1958-present CO2 data is from https://www.esrl.noaa.gov/gmd/webdata/ccgg/trends/co2/co2_annmean_mlo.txt.
For monthly data see
(measurements at Mauna Loa
1850-1958 CO2 data is from http://data.giss.nasa.gov/modelforce/ghgases/Fig1A.ext.txt (ice cores).
1800-1850 CO2 data is from https://www1.ncdc.noaa.gov/pub/data/paleo/icecore/antarctica/law/law_co2.txt (Law Dome ice cores, 75 year smoothed).
1984-present CH4 data is from https://www.esrl.noaa.gov/gmd/webdata/ccgg/trends/ch4/ch4_annmean_gl.txt except that the 2021 figure is preliminary: the average of NOAA's June thru September seasonally-adjusted monthly Mauna Loa measurements.
1800-1992 CH4 data is from https://cdiac.ess-dive.lbl.gov/ftp/trends/atm_meth/EthCH498B.txt (ice cores), interpolated.
The CO2 and CH4 figures are annual averages.
We also have individual graphs of carbon dioxide (CO2) and methane (CH4) levels.