scaling y-axes while using facet_wrap in ggplot - r

Question

I am trying to combine different y-axes while using facet_wrap, but I want all the y-axes to have exactly 3 breaks based in the min and max of that scale, so 0, mid-point, and maximum. It works for some of the plots on the facet grid but not all of them.
Here is the current code:
# data table, in long format
dt = structure(list(Grp = c(rep("GroupA",12),rep("GroupB",12),rep("GroupC",12),rep("GroupD",12)), Type = c(rep(c(rep("Type1",5),rep("Type2",7)),4)), XVal = c(2L, 3L, 4L, 5L, 6L,
1L, 2L, 3L, 4L, 5L, 6L, 7L, 2L, 3L, 4L, 5L, 6L, 1L, 2L, 3L, 4L,
5L, 6L, 7L, 2L, 3L, 4L, 5L, 6L, 1L, 2L, 3L, 4L, 5L, 6L, 7L, 2L,
3L, 4L, 5L, 6L, 1L, 2L, 3L, 4L, 5L, 6L, 7L), YVal = c(0.2417, 0.2156, 0.264,
0.2805, 0.2414, 0.2882, 0.0825, 0.0561, 0.1443, 0.1074, 0.0252,
1e-04, 0.0186, 0.0157, 0.0473, 0.13, 0.1205, 0.0689, 0.1506,
0.2945, 0.3098, 0.1474, 0.3408, 0.1327, 0.0102, 0.0033, 0.0021,
2e-04, 0, 0.0124, 0.0053, 0.0039, 0.0014, 4e-04, 0, 0, 0.0574,
0.1003, 0.0687, 0.0976, 0.1067, 0.1161, 0.0964, 0.0517, 0.0658,
0.0654, 0.0241, 0.0021)), row.names = c(NA,-48L), class = "data.frame")
# first created a function to define three breaks, and round to 2 decimal points
my_breaks <- function(y) {round(seq(0, max(y),length.out = 3),2)}
# use that function in the 'scale_y_continuous' while specifying that the y scale is "free" in facet_wrap
ggplot(dt,aes(x=XVal,y=YVal)) + geom_line(aes(color=Type)) +
facet_wrap(~Grp,scales = "free_y", ncol = 2) +
scale_y_continuous(breaks = my_breaks)
The breaks are what I want for Groups A & D but not B & C. Any help would be greatly appreciated.

Answer 1

It looks like it has something to do with how the rounding is affecting the results of your my_breaks function. If you remove the round so your function looks like
my_breaks <- function(y) {seq(0, max(y),length.out = 3)}
Each facet has exactly three evenly spaced breaks. Now we can format the numbers in the plot:
ggplot(dt,aes(x=XVal,y=YVal)) + geom_line(aes(color=Type)) +
facet_wrap(~Grp,scales = "free_y", ncol = 2) +
scale_y_continuous(breaks = my_breaks,
labels = function(x){round(x,2)})
Notice, however that in Group C, the labels end up not making total sense, since both values for the breaks (0.013 and 0.006) round to 0.01.