# Chapter 15 Advanced usage of chordDiagram()

The default style of chordDiagram() is somehow enough for most visualization tasks, still you can have more configurations on the plot.

The usage is same for both ajacency matrix and ajacency list, so we only demonstrate with the matrix.

## 15.1 Organization of tracks

By default, chordDiagram() creates two tracks, one track for labels and one track for grids with axes.

chordDiagram(mat)
circos.info()
## All your sectors:
## [1] "S1" "S2" "S3" "E1" "E2" "E3" "E4" "E5" "E6"
##
## All your tracks:
## [1] 1 2
##
## Your current sector.index is E6
## Your current track.index is 2

These two tracks can be controlled by annotationTrack argument. Available values for this argument are grid, name and axis. The height of annotation tracks can be set by annotationTrackHeight which is the percentage to the radius of unit circle and can be set by mm_h() function with an absolute unit. Axes are only added if grid is set in annotationTrack (Figure 15.1).

chordDiagram(mat, grid.col = grid.col, annotationTrack = "grid")
chordDiagram(mat, grid.col = grid.col, annotationTrack = c("name", "grid"),
annotationTrackHeight = c(0.03, 0.01))
chordDiagram(mat, grid.col = grid.col, annotationTrack = NULL)

Several empty tracks can be allocated before Chord diagram is drawn. Then self-defined graphics can be added to these empty tracks afterwards. The number of pre-allocated tracks can be set through preAllocateTracks.

chordDiagram(mat, preAllocateTracks = 2)
circos.info()
## All your sectors:
## [1] "S1" "S2" "S3" "E1" "E2" "E3" "E4" "E5" "E6"
##
## All your tracks:
## [1] 1 2 3 4
##
## Your current sector.index is E6
## Your current track.index is 4

The default settings for pre-allocated tracks are:

list(ylim = c(0, 1),
track.height = circos.par("track.height"),
bg.col = NA,
bg.border = NA,
bg.lty = par("lty"),
bg.lwd = par("lwd"))

The default settings for pre-allocated tracks can be overwritten by specifying preAllocateTracks as a list.

chordDiagram(mat, annotationTrack = NULL,
preAllocateTracks = list(track.height = 0.3))

If more than one tracks need to be pre-allocated, just specify preAllocateTracks as a list which contains settings for each track:

chordDiagram(mat, annotationTrack = NULL,
preAllocateTracks = list(list(track.height = 0.1),
list(bg.border = "black")))

By default chordDiagram() provides poor supports for customization of sector labels and axes, but with preAllocateTracks it is rather easy to customize them. Such customization will be introduced in next section.

## 15.2 Customize sector labels

In chordDiagram(), there is no argument to control the style of sector labels, but this can be done by first pre-allocating an empty track and customizing the labels in it later. In the following example, one track is firstly allocated and a Chord diagram is added without label track and axes. Later, the first track is updated with adding labels with clockwise facings (Figure 15.2).

chordDiagram(mat, grid.col = grid.col, annotationTrack = "grid",
preAllocateTracks = list(track.height = max(strwidth(unlist(dimnames(mat))))))
# we go back to the first track and customize sector labels
circos.track(track.index = 1, panel.fun = function(x, y) {
circos.text(CELL_META$xcenter, CELL_META$ylim[1], CELL_META\$sector.index,
facing = "clockwise", niceFacing = TRUE, adj = c(0, 0.5))
}, bg.border = NA) # here set bg.border to NA is important

In the following example, the labels are put on the grids (Figure 15.3). Please note circos.text() and get.cell.meta.data() can be used outside panel.fun if the sector index and track index are specified explicitly.

chordDiagram(mat, grid.col = grid.col,
annotationTrack = c("grid", "axis"), annotationTrackHeight = mm_h(5))
for(si in get.all.sector.index()) {
xlim = get.cell.meta.data("xlim", sector.index = si, track.index = 1)
ylim = get.cell.meta.data("ylim", sector.index = si, track.index = 1)
circos.text(mean(xlim), mean(ylim), si, sector.index = si, track.index = 1,
facing = "bending.inside", niceFacing = TRUE, col = "white")
}

For the last example in this section, if the width of the sector is less than 10 degree, the labels are added in the radical direction (Figure 15.4).

set.seed(123)
mat2 = matrix(rnorm(100), 10)
chordDiagram(mat2, annotationTrack = "grid",
preAllocateTracks = list(track.height = max(strwidth(unlist(dimnames(mat))))))
circos.track(track.index = 1, panel.fun = function(x, y) {
xlim = get.cell.meta.data("xlim")
xplot = get.cell.meta.data("xplot")
ylim = get.cell.meta.data("ylim")
sector.name = get.cell.meta.data("sector.index")

if(abs(xplot[2] - xplot[1]) < 10) {
circos.text(mean(xlim), ylim[1], sector.name, facing = "clockwise",
niceFacing = TRUE, adj = c(0, 0.5), col = "red")
} else {
circos.text(mean(xlim), ylim[1], sector.name, facing = "inside",
niceFacing = TRUE, adj = c(0.5, 0), col= "blue")
}
}, bg.border = NA)

When you set direction of sector labels as radical (clockwise or reverse.clockwise), if the labels are too long and exceed your figure region, you can either decrease the size of the font or set canvas.xlim and canvas.ylim parameters in circos.par() to wider intervals.

## 15.3 Customize sector axes

Axes are helpful to correspond to the absolute values of links. By default chordDiagram() adds axes on the grid track. But it is easy to customize one with self-defined code.

In following example code, we draw another type of axes which show relative percent on sectors. We first pre-allocate an empty track by preAllocateTracks and come back to this track to add axes later.

You may see we add the first axes to the top of second track. You can also put them to the bottom of the first track.

# similar as the previous example, but we only plot the grid track
chordDiagram(mat, grid.col = grid.col, annotationTrack = "grid",
preAllocateTracks = list(track.height = mm_h(5)))
for(si in get.all.sector.index()) {
circos.axis(h = "top", labels.cex = 0.3, sector.index = si, track.index = 2)
}

Now we go back to the first track to add the second type of axes and sector names. In panel.fun, if the sector is less than 30 degree, the break for the axis is set to 0.5 (Figure 15.5).

# the second axis as well as the sector labels are added in this track
circos.track(track.index = 1, panel.fun = function(x, y) {
xlim = get.cell.meta.data("xlim")
ylim = get.cell.meta.data("ylim")
sector.name = get.cell.meta.data("sector.index")
xplot = get.cell.meta.data("xplot")

circos.lines(xlim, c(mean(ylim), mean(ylim)), lty = 3) # dotted line
by = ifelse(abs(xplot[2] - xplot[1]) > 30, 0.2, 0.5)
for(p in seq(by, 1, by = by)) {
circos.text(p*(xlim[2] - xlim[1]) + xlim[1], mean(ylim) + 0.1,
paste0(p*100, "%"), cex = 0.3, adj = c(0.5, 0), niceFacing = TRUE)
}

circos.text(mean(xlim), 1, sector.name, niceFacing = TRUE, adj = c(0.5, 0))
}, bg.border = NA)
circos.clear()

## 15.4 Put horizontally or vertically symmetric

In Chord diagram, when there are two groups (which correspond to rows and columns if the input is an adjacency matrix), it is always visually beautiful to rotate the diagram to be symmetric on horizontal direction or vertical direction. See following example:

par(mfrow = c(1, 2))
circos.par(start.degree = 0)
chordDiagram(mat, grid.col = grid.col, big.gap = 20)
abline(h = 0, lty = 2, col = "#00000080")
circos.clear()

circos.par(start.degree = 90)
chordDiagram(mat, grid.col = grid.col, big.gap = 20)
abline(v = 0, lty = 2, col = "#00000080")
circos.clear()

## 15.5 Compare two Chord diagrams

Normally, in Chord diagram, values in mat are normalized to the summation of the absolute values in the matrix, which means the width for links only represents relative values. Then, when comparing two Chord diagrams, it is necessary that unit width of links in the two plots should be represented in a same scale. This problem can be solved by adding larger gaps to the Chord diagram which has smaller matrix.

First we make the “big” Chord diagram.

mat1 = matrix(sample(20, 25, replace = TRUE), 5)
chordDiagram(mat1, directional = 1, grid.col = rep(1:5, 2), transparency = 0.5,
big.gap = 10, small.gap = 1) # 10 and 1 are default values for the two arguments

The second matrix only has half the values in mat1.

mat2 = mat1 / 2

calc_gap() can be used to calculate the gap for the second Chord diagram to make the scale of the two Chord diagram the same.

gap = calc_gap(mat1, mat2, big.gap = 10, small.gap = 1)
chordDiagram(mat2, directional = 1, grid.col = rep(1:5, 2), transparency = 0.5,
big.gap = gap, small.gap = 1)

Now the scale of the two Chord diagrams (Figure 15.7) are the same if you compare the scale of axes in the two diagrams.

To correctly use the functionality of calc_gap(), the two Chord diagram should have same value for small.gap and there should be no overlap between the two sets of the sectors.

## 15.6 Multiple-group Chord diagram

From verion 0.4.10 of the circlize package, there is a new group argument in chordDiagram() function which is very convenient for making multiple-group Chord diagrams.

I first generate a random matrix where there are three groups (A, B, and C). Note this new functionality works the same for the input as a data frame.

mat1 = matrix(rnorm(25), nrow = 5)
rownames(mat1) = paste0("A", 1:5)
colnames(mat1) = paste0("B", 1:5)

mat2 = matrix(rnorm(25), nrow = 5)
rownames(mat2) = paste0("A", 1:5)
colnames(mat2) = paste0("C", 1:5)

mat3 = matrix(rnorm(25), nrow = 5)
rownames(mat3) = paste0("B", 1:5)
colnames(mat3) = paste0("C", 1:5)

mat = matrix(0, nrow = 10, ncol = 10)
rownames(mat) = c(rownames(mat2), rownames(mat3))
colnames(mat) = c(colnames(mat1), colnames(mat2))
mat[rownames(mat1), colnames(mat1)] = mat1
mat[rownames(mat2), colnames(mat2)] = mat2
mat[rownames(mat3), colnames(mat3)] = mat3
mat
##            B1         B2          B3         B4         B5          C1
## A1  0.9769734  3.2410399  0.51686204  2.1284519  0.4365235 -0.86551286
## A2 -0.3745809 -0.4168576  0.36896453 -0.7413361 -0.4583653 -0.23627957
## A3  1.0527115  0.2982276 -0.21538051 -1.0959963 -1.0633261 -0.19717589
## A4 -1.0491770  0.6365697  0.06529303  0.0377884  1.2631852  1.10992029
## A5 -1.2601552 -0.4837806 -0.03406725  0.3104807 -0.3496504  0.08473729
## B1  0.0000000  0.0000000  0.00000000  0.0000000  0.0000000  1.65090747
## B2  0.0000000  0.0000000  0.00000000  0.0000000  0.0000000 -0.05402813
## B3  0.0000000  0.0000000  0.00000000  0.0000000  0.0000000  0.11924524
## B4  0.0000000  0.0000000  0.00000000  0.0000000  0.0000000  0.24368743
## B5  0.0000000  0.0000000  0.00000000  0.0000000  0.0000000  1.23247588
##             C2         C3          C4          C5
## A1  0.75405379 -0.8953634 -0.61116592  0.54319406
## A2 -0.49929202 -1.3108015 -1.18548008 -0.41433995
## A3  0.21444531  1.9972134  2.19881035 -0.47624689
## A4 -0.32468591  0.6007088  1.31241298 -0.78860284
## A5  0.09458353 -1.2512714 -0.26514506 -0.59461727
## B1 -0.51606383 -1.2362731  0.70758835  0.88465050
## B2 -0.99250715 -1.2847157 -0.36365730 -1.01559258
## B3  1.67569693 -0.5739735  0.05974994  1.95529397
## B4 -0.44116322  0.6179858 -0.70459646 -0.09031959
## B5 -0.72306597  1.1098481 -0.71721816  0.21453883

The main thing is to create “a grouping variable”. The variable contains the group labels and the sector names are used as the names in the vector.

nm = unique(unlist(dimnames(mat)))
group = structure(gsub("\\d", "", nm), names = nm)
group
##  A1  A2  A3  A4  A5  B1  B2  B3  B4  B5  C1  C2  C3  C4  C5
## "A" "A" "A" "A" "A" "B" "B" "B" "B" "B" "C" "C" "C" "C" "C"

Assign group variable to the group argument:

grid.col = structure(c(rep(2, 5), rep(3, 5), rep(4, 5)),
names = c(paste0("A", 1:5), paste0("B", 1:5), paste0("C", 1:5)))
chordDiagram(mat, group = group, grid.col = grid.col)
circos.clear()

We can try another grouping:

group = structure(gsub("^\\w", "", nm), names = nm)
group
##  A1  A2  A3  A4  A5  B1  B2  B3  B4  B5  C1  C2  C3  C4  C5
## "1" "2" "3" "4" "5" "1" "2" "3" "4" "5" "1" "2" "3" "4" "5"
chordDiagram(mat, group = group, grid.col = grid.col)
circos.clear()

The order of group controls the sector orders and if group is set as a factor, the order of levels controls the order of groups.

group = structure(gsub("\\d", "", nm), names = nm)
group = factor(group[sample(length(group), length(group))], levels = c("C", "A", "B"))
group
## B3 A3 B5 B2 C1 A1 A2 C4 A5 C2 C5 B4 A4 B1 C3
##  B  A  B  B  C  A  A  C  A  C  C  B  A  B  C
## Levels: C A B
chordDiagram(mat, group = group, grid.col = grid.col)
circos.clear()

The gap between groups is controlled by big.gap argument and the gap between sectors is controlled by small.gap argument.

group = structure(gsub("\\d", "", nm), names = nm)
chordDiagram(mat, group = group, grid.col = grid.col, big.gap = 20, small.gap = 5)
circos.clear()

As a normal Chord diagram, the labels and other tracks can be manually adjusted:

group = structure(gsub("\\d", "", nm), names = nm)
chordDiagram(mat, group = group, grid.col = grid.col,
annotationTrack = c("grid", "axis"),
preAllocateTracks = list(
track.height = mm_h(4),
track.margin = c(mm_h(4), 0)
))
circos.track(track.index = 2, panel.fun = function(x, y) {
sector.index = get.cell.meta.data("sector.index")
xlim = get.cell.meta.data("xlim")
ylim = get.cell.meta.data("ylim")
circos.text(mean(xlim), mean(ylim), sector.index, cex = 0.6, niceFacing = TRUE)
}, bg.border = NA)

highlight.sector(rownames(mat1), track.index = 1, col = "red",
text = "A", cex = 0.8, text.col = "white", niceFacing = TRUE)
highlight.sector(colnames(mat1), track.index = 1, col = "green",
text = "B", cex = 0.8, text.col = "white", niceFacing = TRUE)
highlight.sector(colnames(mat2), track.index = 1, col = "blue",
text = "C", cex = 0.8, text.col = "white", niceFacing = TRUE)
circos.clear()

The implementation of multi-group Chord diagram is very simple. It can also be done by setting a proper sector order (by order argument) and a gap.after parameter. The previous example can be implemented without the group argument by the following code, but you need to manually calculate the proper value for gap.after. Setting group argument automatically does this for you.

circos.par(gap.after = c(rep(1, 4), 5, rep(1, 4), 5, rep(1, 4), 5))
chordDiagram(mat, order = names(sort(group)), grid.col = grid.col)
circos.clear()