Facet labelling with R

Question

I am trying to plots subsets of data using facet plot. It is 2 columns x4 row plots. First column with values between 120 and 150, and second column with values between 180 and 250, and the variables are assigned as Lesser or Greater in datafile. I am facing problem with adding of specific labels ('120 <= greek symbol(alpha) <= 150', '180 <= greek symbol(alpha) <= 250) to the column variables ("Lesser", "Greater"). I tried doing it as: ``` require(graphics) library(ggplot2) hp <- ggplot2(data) + .... xlow <- paste("120 <", expression(alpha), " < 150") xhi <- paste("180 <", expression(alpha), " < 250") mf_labeller <- function(var, value){ value <- as.character(value) if (var=="regime") { value[value=="Lesser"] <- xlow value[value=="Greater"] <- xhi } return(value) } (hp %+% data) + facet_grid(param~regime, scales="free", labeller=mf_labeller) ``` I get "Error in value == "Greater" : comparison is not allowed for expressions". I tried coercing the data too. If I do the labelling as in a…

Accepted Answer

The form you want for the plotmath expression is

```
expression(120 <= alpha~phantom() <= 150)
expression(180 <= alpha~phantom() <= 250)
```

which you can get that way or indirectly by

```
parse(text="120 <= alpha~phantom() <= 150")
parse(text="180 <= alpha~phantom() <= 250")
```

The easiest way is to set the levels of the factor to those strings and use the `label_parsed` function.

Mocking this up with the `mtcars` data, since you didn't provide a reproducible example:

```
dat <- mtcars   
dat$regime <- factor(dat$am)
levels(dat$regime) <- list("120 <= alpha~phantom() <= 150" = "0",
                           "180 <= alpha~phantom() <= 250" = "1")

ggplot(aes(x=wt, y=mpg), data=dat) +
  geom_point() +
  facet_grid(cyl ~ regime, labeller=label_parsed)
```

If you really want to use your own labeller, and don't want to change the factor levels of your underlying data, you can do that too. Start with mock data with "Lesser" and "Greater" levels:

```
dat <- mtcars   
dat$regime <- factor(dat$am, levels=c(0,1), labels=c("Lesser", "Greater"))
```

Then the labeller function and ggplot code would be:

```
mf_labeller <- function(var, value){
  value <- as.character(value)
  if (var=="regime") {
    value[value=="Lesser"] <- "120 <= alpha~phantom() <= 150"
    value[value=="Greater"]  <- "180 <= alpha~phantom() <= 250"
    value <- lapply(value, function(x) parse(text=x))
  }
  return(value) 
}

ggplot(aes(x=wt, y=mpg), data=dat) +
  geom_point() +
  facet_grid(cyl ~ regime, labeller=mf_labeller)
```

The graph looks the same.

Facet labelling with R

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