Trouble with ggplot2 multiple legend alignment with grid.arrange - r

Question

I'm trying to add two legends to the same ggplot2 graph and I'm having a terrible time with the alignment. I've got points representing some data and then fitted regression lines as well, so I want the legend for colors to be split up so that it's clear which are just for the points and which are for the fitted lines. Here's my best attempt so far:
library(ggplot2)
library(gridExtra)
StudyResults <- data.frame(TreatmentArm = rep(c("A", "B"), each = 10),
SubjectID = rep(1:10, each = 2),
Glucose = rnorm(20, 50, 10),
Insulin = rnorm(20, 0.15, 0.05),
StudyDay = rep(c("SD1", "SD2"), 10))
Trend <- data.frame(Gender = rep(c("F", "M"), each = 50),
Glucose = seq(20, 80, length = 50),
Insulin = NA)
Trend$Insulin[Trend$Gender == "F"] <- 2/Trend$Glucose[Trend$Gender == "F"]
Trend$Insulin[Trend$Gender == "M"] <- 5/Trend$Glucose[Trend$Gender == "M"]
PlotTrend <- ggplot(Trend, aes(x = Glucose, y = Insulin, color = Gender)) +
geom_line() + scale_color_manual(values = c("red", "blue"))
PlotStudy <- ggplot(StudyResults, aes(x = Glucose, y = Insulin, shape = StudyDay,
color = TreatmentArm, group = SubjectID)) +
geom_point() + geom_line() +
scale_color_manual(values = c("green", "black"))
g_legend <- function(a.gplot){
tmp <- ggplot_gtable(ggplot_build(a.gplot))
leg <- which(sapply(tmp$grobs, function(x) x$name) == "guide-box")
legend <- tmp$grobs[[leg]]
return(legend)}
LegendTrend <- g_legend(PlotTrend)
LegendStudy <- g_legend(PlotStudy)
PlotMain <- ggplot(StudyResults, aes(x = Glucose, y = Insulin, shape = StudyDay,
color = TreatmentArm, group = SubjectID)) +
geom_point() + geom_line() +
scale_color_manual(values = c("green", "black")) +
geom_line(data = Trend[Trend$Gender == "F", ],
aes(x = Glucose, y = Insulin),
inherit.aes = FALSE, color = "red") +
geom_line(data = Trend[Trend$Gender == "M", ],
aes(x = Glucose, y = Insulin),
inherit.aes = FALSE, color = "blue") +
theme(legend.position = "none")
grid.arrange(PlotMain,
arrangeGrob(LegendStudy, LegendTrend, nrow = 2,
heights = c(unit(0.5, "npc"),
unit(0.5, "npc")),
widths = unit(0.5, "npc")),
ncol = 2, widths = c(10, 3))
But the positioning of the legends is TERRIBLE:
I don't know how to make the legend for "Gender" be aligned on the left with the other two, and I don't want there to be so much white space in the middle.
I also fiddled with gtable and grid packages' commands like viewPort but am completely clueless about how to use them. (Suggestions for decent tutorials would be much appreciated; I haven't ever found any.) I tried
library(gtable)
library(grid)
grid.newpage()
pushViewport(vp = viewport())
vp1 <- viewport(width = 0.8, height = 1, x = 0, y = 0)
grid.draw(PlotMain)
vp2 <- viewport(width = 0.2, x = 0.9, y = 0.6)
pushViewport(vp2)
grid.draw(LegendStudy)
pushViewport(vp3 = viewport(width = 0.2, x = 0.9, y = 0.4))
grid.draw(LegendTrend)
but I clearly have no idea how to use this because the legends overlapped the main plot and were positioned in what seems to me a completely random (i.e., unrelated to the x and y coordinates I thought I was specifying) way.

Answer 1

it's unclear what the OP wants but the next option would be adding the second legend's gtable to the first,
library(gtable)
leg2 <- LegendTrend$grobs[[1]]
leg <- gtable_add_rows(LegendStudy, pos = nrow(LegendStudy) - 1,
heights = sum(leg2$heights))
leg <- gtable_add_grob(leg, leg2, t = nrow(leg) - 1, l = 3)
grid.arrange(PlotMain, right = leg)
For a tutorial on grid viewports, there's the R graphics book, but ggplot2's design has drifted substantially from base grid with the introduction of gtable as an intermediate framework to place graphic elements. ggplot2 legends are a complex structure of nested gtables, that few people understand. gtable is not documented, and its development stopped early on, so the best source of information is the code itself.

Answer 2

leg <- gtable_rbind(LegendStudy, LegendTrend, size = "first")
grid.arrange(PlotMain, right = leg)