To see precise values, hover your mouse cursor over the blue graph trace. (However, the ice core values are less accurate than the Mauna Loa measurements.)
1840-1992 data (in light blue) is from http://cdiac.ornl.gov/ftp/trends/atm_meth/EthCH498B.txt (ice cores), interpolated.
1984-present data (in dark blue) is from ftp://aftp.cmdl.noaa.gov/products/trends/ch4/ch4_annmean_gl.txt (measurements at Mauna Loa Observatory, HI).
We also have a similar graph for carbon dioxide (CO2), and a combined graph with both CO2 and CH4.