How can I add a geom_quantile legend to this customized facet_wrap?

Question

I recently received great help from SO in producing this customized facet_wrap-plot shown below.

Question: how can I add the dotted linetype used in geom_quantile(linetype=2) as legend with the text "50th percentile"?

I have sought solutions in similar questions on SO, but my question has not been answered.

My current plot looks like this

Produced with the following code

cols = c("#E1B930", "#2C77BF","#E38072","#6DBCC3")
ggplot(p, aes(x=n.fjernet,y=os.neck)) + geom_point(aes(color=uiccc),shape=20, size=5,alpha=0.7) + 

  geom_quantile(quantiles = 0.5,col="black", size=1,linetype=2, show.legend = F) + facet_wrap(.~factor(uiccc)) +

  scale_fill_manual(values=cols) + 

  scale_colour_manual(values=cols)  +

  scale_x_continuous(breaks = seq(0,50, by=10), name="Lymph nodal yield") +
  scale_y_continuous(name="Time to death (months)") +

  theme(strip.background = element_blank(),
        strip.text = element_text(color = "transparent"),
        axis.title.x = element_text(color = "grey20", size = 14, face="bold", margin=ggplot2::margin(t=10)),
        axis.title.y = element_text(color = "grey20", size = 14, face="bold", margin=ggplot2::margin(r=10)),
        legend.position="none",
        plot.margin = unit(c(1,3,1,1), "lines")) +
        coord_cartesian(clip = "off",ylim = c(0,175)) +
        geom_text(data = . %>% distinct(uiccc), 
        aes(label = factor(uiccc), color = uiccc), y = 190, x = 30, hjust = 0.5, fontface = "bold",cex=5)

I would like the plot to have a legend that indicates the "50th percentile" as demonstrated by the linetype=2 in geom_quantile(), exclusively, which looks like this (manually added in photoshop):

First: I have removed theme(legend.position="none")

Second: I have added show.legend=TRUE in geom_quantile

Third: I have added show.legend=FALSE in geom_point

Unfortunately, these edits does not produce the requested legend:

How can I proceed?

My data p

p <- structure(list(n.fjernet = c(18L, 11L, 14L, 15L, 9L, 6L, 3L, 
16L, 4L, 6L, 10L, 13L, 33L, 16L, 6L, 9L, 23L, 9L, 8L, 13L, 5L, 
30L, 25L, 3L, 9L, 9L, 12L, 7L, 38L, 5L, 7L, 15L, 4L, 6L, 15L, 
9L, 8L, 7L, 4L, 6L, 10L, 8L, 4L, 9L, 10L, 14L, 14L, 3L, 4L, 6L, 
6L, 20L, 3L, 26L, 13L, 13L, 13L, 13L, 3L, 7L, 6L, 5L, 10L, 15L, 
29L, 7L, 6L, 11L, 17L, 14L, 18L, 22L, 9L, 20L, 34L, 9L, 8L, 8L, 
11L, 3L, 4L, 4L, 5L, 3L, 2L, 8L, 5L, 18L, 7L, 9L, 13L, 18L, 19L, 
14L, 46L, 23L, 11L, 6L, 18L, 20L, 4L, 2L, 7L, 7L, 4L, 11L, 13L, 
13L, 9L, 9L, 9L, 12L, 11L, 16L, 6L, 13L, 8L, 17L, 5L, 8L, 22L, 
19L, 3L, 15L, 14L, 7L, 18L, 9L, 10L, 18L, 24L, 11L, 15L, 7L, 
6L, 4L, 24L, 23L, 8L, 20L, 9L, 22L, 11L, 2L, 24L, 15L, 5L, 8L, 
11L, 11L, 11L, 15L, 6L, 16L, 7L, 9L, 16L, 11L, 33L, 27L, 16L, 
57L, 5L, 7L, 8L, 11L, 15L, 15L, 12L, 5L, 9L, 49L, 11L, 28L, 19L, 
13L, 23L, 11L, 12L, 10L, 4L, 14L, 6L, 12L, 32L, 13L, 12L, 4L, 
11L, 17L, 10L, 5L, 15L, 21L, 19L, 11L, 31L, 9L, 20L, 11L, 16L, 
12L, 6L, 16L, 27L, 30L, 18L, 18L, 10L, 7L, 23L, 16L, 15L, 4L, 
12L, 9L, 10L, 11L, 7L, 8L, 8L, 7L, 6L, 9L, 9L, 13L, 15L, 12L, 
35L, 12L, 5L, 19L, 27L, 34L, 10L, 16L, 18L, 6L, 22L), os.neck = c(11.5, 
74.38, 17.02, 7.89, 96.03, 40.48, 17.74, 14.65, 62.46, 12.55, 
9.92, 26.05, 45.47, 17.38, 39.72, 51.45, 8.61, 76.98, 67.09, 
94.79, 72.15, 93.93, 17.05, 12.48, 91.6, 15.87, 11.04, 67.22, 
67.02, 8.94, 6.6, 5.09, 10.68, 17.15, 0.07, 5.19, 40.77, 0.2, 
170.88, 5.55, 1.61, 38.28, 10.58, 32.99, 110.98, 103.69, 122.32, 
14.78, 42.74, 4.04, 8.28, 84.96, 11.7, 49.97, 120.48, 52.6, 71.26, 
16.3, 100.14, 55.03, 6.51, 89.89, 51.71, 24.97, 55.66, 21.91, 
81.48, 30.92, 1.58, 7.52, 30.75, 3.45, 19.22, 5.42, 17.68, 45.54, 
76.22, 125.34, 83.62, 30.82, 90.32, 1.84, 19.98, 20.53, 32.59, 
54.77, 2.3, 106.84, 22.28, 45.18, 4.47, 39.66, 32.3, 16.23, 3.88, 
2.23, 0.23, 18.73, 0.79, 28.75, 79.54, 14.46, 15.15, 54.97, 48.59, 
34.83, 58.42, 35.29, 45.73, 57.53, 63.11, 65.05, 29.54, 77.21, 
63.48, 83.35, 34.3, 64.49, 29.54, 62.69, 21.62, 49.35, 99.02, 
15.8, 41.89, 12.98, 13.8, 43.6, 57.23, 31.38, 70.74, 39.46, 20.76, 
67.22, 127.15, 74.12, 1.97, 7.39, 25.17, 28.22, 14, 36.53, 20.83, 
19.55, 40.77, 27.76, 45.31, 34.46, 35.55, 26.94, 9.43, 10.51, 
6.8, 8.18, 8.02, 14.29, 6.11, 13.8, 4.9, 4.04, 14.82, 11.66, 
73.07, 92.91, 99.98, 10.64, 10.05, 95.8, 7.23, 12.81, 43.99, 
13.9, 10.25, 16.36, 18.2, 18.76, 12.32, 8.64, 11.79, 112.04, 
70.97, 31.28, 28.85, 21.49, 19.94, 22.14, 29.44, 67.62, 11.01, 
45.24, 110.72, 20.24, 14.06, 12.88, 31.51, 8.08, 13.08, 21.45, 
24.28, 21.98, 32.89, 23.26, 15.41, 15.41, 13.8, 40.12, 8.02, 
15.77, 49.81, 18.17, 24.21, 47.08, 6.6, 37.16, 13.01, 8.38, 14.36, 
18.27, 17.28, 73.76, 68.21, 22.83, 2.66, 69.06, 17.05, 8.61, 
23.33, 13.34, 12.65, 8.77, 128.92, 16.1, 4.99, 11.73, 22.97, 
40.12, 20.37, 2.04, 45.73), uiccc = structure(c(4L, 3L, 3L, 2L, 
2L, 2L, 2L, 4L, 1L, 1L, 2L, 1L, 4L, 2L, 1L, 2L, 3L, 1L, 2L, 3L, 
2L, 1L, 2L, 3L, 2L, 4L, 1L, 1L, 2L, 4L, 4L, 1L, 3L, 3L, 4L, 3L, 
1L, 4L, 2L, 3L, 4L, 4L, 4L, 3L, 2L, 4L, 1L, 4L, 2L, 4L, 4L, 2L, 
4L, 4L, 1L, 4L, 2L, 3L, 2L, 2L, 3L, 2L, 4L, 4L, 2L, 2L, 3L, 1L, 
4L, 4L, 4L, 4L, 4L, 3L, 2L, 2L, 2L, 2L, 2L, 1L, 1L, 2L, 1L, 1L, 
1L, 1L, 4L, 2L, 4L, 1L, 2L, 1L, 1L, 3L, 3L, 4L, 4L, 4L, 4L, 4L, 
4L, 2L, 3L, 3L, 4L, 1L, 1L, 3L, 1L, 4L, 2L, 1L, 3L, 1L, 2L, 1L, 
1L, 4L, 1L, 1L, 4L, 1L, 1L, 3L, 2L, 2L, 1L, 4L, 4L, 4L, 4L, 1L, 
1L, 1L, 2L, 2L, 4L, 4L, 2L, 3L, 4L, 2L, 4L, 1L, 1L, 3L, 3L, 1L, 
1L, 3L, 4L, 4L, 2L, 4L, 4L, 3L, 4L, 4L, 4L, 4L, 4L, 4L, 3L, 2L, 
2L, 4L, 3L, 1L, 4L, 3L, 4L, 4L, 3L, 1L, 4L, 4L, 4L, 4L, 2L, 2L, 
4L, 4L, 1L, 4L, 4L, 2L, 4L, 4L, 4L, 3L, 4L, 3L, 3L, 4L, 4L, 2L, 
4L, 4L, 2L, 4L, 4L, 4L, 4L, 1L, 4L, 4L, 3L, 4L, 4L, 4L, 4L, 4L, 
4L, 4L, 4L, 4L, 4L, 2L, 3L, 1L, 2L, 1L, 2L, 2L, 4L, 4L, 4L, 4L, 
4L, 4L, 1L, 3L, 4L, 4L, 1L, 3L, 3L, 4L, 3L), .Label = c("UICC Stage I", 
"UICC Stage II", "UICC Stage III", "UICC Stage IV"), class = "factor")), row.names = c(NA, 
-239L), class = "data.frame")

Answer 1

One possible solution is to specify a "fake" color title in the aes of geom_quantile.

Then, you can remove legend for points and labels by adding show.legend = FALSE into their respective geom and manipulate breaks in scale_color_manual to show only the color attribute for your 50th percentile category. Finally, manipulating various parameters for legend in theme will get you to what you desired.

Altogether, you can have a code like this (PS: scale_fill_manual is useless in your current example):

ggplot(p, aes(x=n.fjernet,y=os.neck)) + 
  geom_point(aes(color=uiccc),shape=20, size=5,alpha=0.7, show.legend = FALSE) + 
  geom_quantile(aes(colour = "50th percentile"), quantiles = 0.5, size=1,linetype=2) + 
  facet_wrap(.~factor(uiccc)) +
  #scale_fill_manual(values=cols) + 
  scale_colour_manual(values=cols, breaks = c("50th percentile"), name = "")  +
  scale_x_continuous(breaks = seq(0,50, by=10), name="Lymph nodal yield") +
  scale_y_continuous(name="Time to death (months)") +
  theme(strip.background = element_blank(),
        strip.text = element_text(color = "transparent"),
        axis.title.x = element_text(color = "grey20", size = 14, face="bold", margin=ggplot2::margin(t=10)),
        axis.title.y = element_text(color = "grey20", size = 14, face="bold", margin=ggplot2::margin(r=10)),
        legend.position="top",
        legend.text=element_text(size=rel(2)),
        legend.key.size = unit(2, "cm"),
        plot.margin = unit(c(1,3,1,1), "lines")) +
  coord_cartesian(clip = "off",ylim = c(0,175)) +
  geom_text(data = . %>% distinct(uiccc), 
            aes(label = factor(uiccc), color = uiccc), y = 190, x = 30, hjust = 0.5, fontface = "bold",cex=5, show.legend = FALSE)