Feature request: geom_mark_polygon

[assaron]

Related to: #321

I have my own algorithm to calculate the border of points group, but would like to use your label location procedure. Do you think it'd be possible to extract something like geom_mark_polygon from the current code?

This is the plot I'm interested to add labels to:

library(ggplot2)
library(ggforce)
library(data.table)

load(gzfile("border_example.rda.gz"))

ggplot(data, aes(x=UMAP_1, y=UMAP_2)) + 
    geom_point(color="grey") + 
    geom_shape(data=maskTable, aes(group=group, color=cluster),
               linewidth=1, fill=NA, expand=unit(-1, "pt")) +
    coord_fixed() + 
    theme_classic()

border_example.rda.gz

Currently, the best I can do is something like this, relying on geom_mark_hull:

ggplot(data, aes(x=UMAP_1, y=UMAP_2)) +
    geom_point(color="grey") +
    geom_shape(data=maskTable, aes(group=group, color=cluster),
               linewidth=1, fill=NA, expand=unit(-1, "pt")) +
    geom_mark_hull(data=maskTable, aes(fill = NA, group=cluster, label = cluster),
                   radius=0, expand=0, con.cap=0) +
    coord_fixed() +
    theme_classic()

However, I don't need to actually build a hull, since I have already have a proper polygon, and I don't want to have an additional dependency.

There are also couple of other questions:

1. Several label connectors (for example, NK, CD8 T, and others) lead to a split point between two clusters, resulting in a bit of ambiguity. Is there a way to fix it?
2. Is there a way to remove fill? fill=NA doesn't seem to work as I would expect.
3. Is there a way to decrease the length of the connectors? They seem to be longer that I'd like, and I couldn't find an option to control it.

[assaron]

Update: I've resolved the additional questions:

library(ggplot2)
library(ggforce)
library(data.table)

load(gzfile("border_example.rda.gz"))
ggplot(data, aes(x=UMAP_1, y=UMAP_2)) +
    geom_point(color="grey") +
    geom_shape(data=maskTable, aes(group=group, color=cluster),
               linewidth=1, fill=NA, expand=unit(-1, "pt")) +
    geom_mark_hull(data=maskTable, fill = NA, color=NA, aes(group=cluster, label = cluster),
                   radius=0, expand=0, con.cap=0, con.type = "straight",
                   label.fontsize = 10, label.buffer = unit(0, "cm"),
                   label.fontface = "plain",
                   label.minwidth = 0,
                   label.margin = margin(2, 2, 2, 2, "pt"),
                   label.lineheight = 0) +
    # expanding to give a bit more space for labels
    scale_x_continuous(expand = expansion(mult = 0.1)) +
    scale_y_continuous(expand = expansion(mult = 0.1)) +
    coord_fixed() +
    theme_classic()

So only the main question remains: to do it without building the unnessary hulls.

[assaron]

However, I don't need to actually build a hull, since I have already have a proper polygon, and I don't want to have an additional dependency.

I realized, that concaveman is no longer a dependency (unfortunately, not yet in a CRAN version), but anyway constructing a hull is unnecessary here, and can result in contour mismatches.

I think I can provide a draft pull request, if the general idea sounds reasonable.