isomeans grid for MMC plots.
panel.isomeans(ybar,
               lty.iso=7,
               col.iso='darkgray',
               lwd.iso=1,
               lty.contr0=2,
               col.contr0='darkgray',
               lwd.contr0=1,
               ...,
               col, lwd, lty ## capture potentially ambiguous name
               )Vector of means.
color, line type, line width for the isomeans grid.
color, line type, line width for the vertical contrast=0 line.
ignore any additional arguments
ignore these arguments.  They are captured here to avoid
ambiguity with col.iso and lty.iso.
See mmc for the references and examples.