R, how to change to interval in the y axis of a forest plot?

Question

I'm preparing e a forest plot in R through the forestplot package. The possible values range from -1 to +1, and I would like that to be the range on my x axis. My values, however, range from 0 to 1, so R is depicting only that part on the x axis.

See the image here:

How can I make the x axis interval range from -1 to +1? I want to leave a complete empty space on the right to show no value is there.

Here's my code:

library("forestplot")

cochrane_from_rmeta <- 
  structure(list(
    lower = c(NA, NA, 0.026, 0.043, 0.184, 0.333,  0.026),
    mean  = c(NA, NA, 0.502, 0.534, 0.548, 0.792,  0.600), 
    upper = c(NA, NA, 0.978, 0.949, 0.911, 0.936,  0.967)),
    .Names = c("mean", "lower", "upper"), 
    row.names = c(NA, -7L), 
    class = "data.frame")

tabletext<-cbind(
  c("", "aaa", "bbb", "ccc", 
    "ddd", "eee",
     "Summary"),
  c("", "#datasets", "1", "2", 
    "3", "4",
     NA))     

png(paste0("forestPlot_",exe_num,".png"))
forestplot(tabletext, 
           cochrane_from_rmeta, 
           new_page = TRUE,
           is.summary=c(TRUE,TRUE,rep(FALSE,4),TRUE),
           clip=c(-1,1), 
           xlog=FALSE, 
           col=fpColors(box="royalblue",line="darkblue", summary="royalblue"))

Answer 1

There doesn't seem to be a direct way of doing this, but if you set a lower and upper of -1 and 1 in your first row while leaving mean as NA, it will fix the x range without affecting the plot otherwise:

cochrane_from_rmeta <- 
  structure(list(
    lower = c(-1, NA, 0.043, 0.043, 0.184, 0.333,  0.026),
    mean  = c(NA, NA, 0.502, 0.534, 0.548, 0.792,  0.600), 
    upper = c(1, NA, 0.978, 0.949, 0.911, 0.936,  0.967)),
    .Names = c("mean", "lower", "upper"), 
    row.names = c(NA, -7L), 
    class = "data.frame")

tabletext<-cbind(
  c("", "aaa", "bbb", "ccc", 
    "ddd", "eee",
    "Summary"),
  c("", "#datasets", "1", "2", 
    "3", "4",
    NA))

forestplot(tabletext, 
           cochrane_from_rmeta, 
           new_page = TRUE,
           is.summary=c(FALSE,TRUE,rep(FALSE,4),TRUE),
           clip=c(-1,1), 
           xlog=FALSE, 
           mar = unit(c(50, 30, -30, 30), "pt"),
           col=fpColors(box="royalblue",line="darkblue", summary="royalblue"), 
           graphwidth = unit(0.7, "npc"))