We can not easily create the dual y axis with ggplot2.
These tricks can not work if figures have multiple panels (created by geom_wrap or geom_grid), e.g. 4 panel figure below. I would like to add the axis of p1 to the right side of panels of merged figure.
EDIT: I removed my codes here and post my own solution as an answer. Just in case some one needs it.
I post my own solution as an answer, just in case some one needs it.
library(ggplot2) library(gtable) library(grid) df1 <- expand.grid(list(x = 1:10, z = 1:4, col = 1:2)) df2 <- df1 df1$y <- df1$x * df1$x + df1$col * 10 df2$y <- 4 * df2$x + df1$col * 10 p1 <- ggplot(df1) p1 <- p1 + geom_point(aes(x, y, colour = factor(col))) p1 <- p1 + facet_wrap(~z) p1 <- p1 + ylab('Y label for figure 1') p1 <- p1 + theme(legend.position = 'bottom', plot.margin = unit(c(1, 2, 0.5, 0.5), 'cm')) p2 <- ggplot(df2) p2 <- p2 + geom_line(aes(x, y, colour = factor(col))) p2 <- p2 + facet_wrap(~z) p2 <- p2 + ylab('Y label for figure 2') p2 <- p2 + theme(legend.position = 'bottom', plot.margin = unit(c(1, 2, 0.5, 0.5), 'cm')) g1 <- ggplot_gtable(ggplot_build(p1)) g2 <- ggplot_gtable(ggplot_build(p2)) combo_grob <- g2 pos <- length(combo_grob) - 1 combo_grob$grobs[[pos]] <- cbind(g1$grobs[[pos]], g2$grobs[[pos]], size = 'first') panel_num <- length(unique(df1$z)) for (i in seq(panel_num)) { # grid.ls(g1$grobs[[i + 1]]) panel_grob <- getGrob(g1$grobs[[i + 1]], 'geom_point.points', grep = TRUE, global = TRUE) combo_grob$grobs[[i + 1]] <- addGrob(combo_grob$grobs[[i + 1]], panel_grob) } pos_a <- grep('axis_l', names(g1$grobs)) axis <- g1$grobs[pos_a] for (i in seq(along = axis)) { if (i %in% c(2, 4)) { pp <- c(subset(g1$layout, name == paste0('panel-', i), se = t:r)) ax <- axis[[1]]$children[[2]] ax$widths <- rev(ax$widths) ax$grobs <- rev(ax$grobs) ax$grobs[[1]]$x <-
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