rolling median in ggplot2 - r

Question

I would like to add rolling medians to my data in ggplot2. Calculating the rolling median in the ggplot aes and in the data.frame itself do not produce similar results (see plots).
I am looking for a solution within ggplot2 that produces the same results as in the data.frame calculation. I know this can be done with ggseas::stat_rollapplyr, but would prefer a solution in base ggplot2.
code;
library(ggplot2)
library(data.table)
library(zoo)
library(gridExtra)
# set up dummy data
set.seed(123)
x = data.table(
date = rep( seq(from = as.Date("2016-01-01"), to = as.Date("2016-04-01"), by = "day"), 2),
y = c(5 + runif(92), 6 + runif(92)),
label = c(rep("A", 92), rep("B", 92))
)
x[, `:=` (
roll = rollmedian(y, k = 15, fill = NA, align = "center")
), by = label]
# plots
theme_set(theme_bw())
p = ggplot(x) +
geom_line(aes(date, y), col = "lightgrey") +
facet_wrap(~label)
# within aes
p1 = p +
geom_line(aes(date, rollmedian(y, k = 15, fill = NA, align = "center"))) +
labs(title = "within aes")
# calculated in data.frame
p2 = p +
geom_line(aes(date, roll)) +
labs(title = "within data.frame")
grid.arrange(p1, p2)
sessionInfo()
R version 3.2.3 (2015-12-10)
Platform: x86_64-pc-linux-gnu (64-bit)
Running under: Ubuntu 16.04.1 LTS
locale:
[1] LC_CTYPE=en_US.UTF-8 LC_NUMERIC=C LC_TIME=nl_NL.UTF-8 LC_COLLATE=en_US.UTF-8 LC_MONETARY=nl_NL.UTF-8 LC_MESSAGES=en_US.UTF-8 LC_PAPER=nl_NL.UTF-8
[8] LC_NAME=C LC_ADDRESS=C LC_TELEPHONE=C LC_MEASUREMENT=nl_NL.UTF-8 LC_IDENTIFICATION=C
attached base packages:
[1] stats graphics grDevices utils datasets methods base
other attached packages:
[1] zoo_1.7-13 magrittr_1.5 data.table_1.9.7 ggplot2_2.1.0.9000
loaded via a namespace (and not attached):
[1] labeling_0.3 colorspace_1.2-6 scales_0.4.0 assertthat_0.1 plyr_1.8.4 rsconnect_0.4.3 tools_3.2.3 gtable_0.2.0 tibble_1.2 Rcpp_0.12.
7 grid_3.2.3 munsell_0.4.3
[13] lattice_0.20-33