Upper panel: mean metallicity of the LAMOST sample as a function of guiding radius. Each point indicates the arithmetic mean within the corresponding bin, with the shaded region showing the uncertainty of the mean. The error bars denote the 1σ dispersion of the data in each bin. Lower panel: rotational velocity as a function of metallicity for the LAMOST sample. The vertical shaded band marks the metallicity dispersion at R_g ~ 13.5 kpc.