Draw several plots at specified coordinates using

I want to place ggplot graphs in specified locations on the map. I chose ggplot2 package because I'm more familiar with it then with grid. If someone will help me with a small example how to use grid for such a task, I will appreciate for such an answer as well.

Here is a simple example:

# create base plot
g <- ggplot(data.frame(x=c(-104,-94), y=c(33,38)), aes(x=x, y=y)) + 
  geom_blank()

# create theme
tm <- theme(axis.title = element_blank(),
            axis.text = element_blank(),
            axis.ticks = element_blank(),
            axis.line = element_blank(),
            panel.background = element_blank(),
            panel.grid = element_blank(),
            panel.border = element_rect(color="grey", fill=NA),
            title = element_text(size=5))

# create two plot which should be placed on the base plot
p1 <- ggplot(data.frame(x=c(-104,-94), y=c(33,38)), aes(x=x, y=y)) + 
  geom_point() + tm
p2 <- ggplot(data.frame(x=c(-100,-98), y=c(34,37)), aes(x=x, y=y)) + 
  geom_point() + tm

# place them using annotation_custom() function
a1 <- annotation_custom(grob = ggplotGrob(p1), 
                        xmin = -104, xmax = -102,
                        ymin = 33, ymax = 35)
a2 <- annotation_custom(grob = ggplotGrob(p2), 
                        xmin = -100, xmax = -98,
                        ymin = 35, ymax = 37)

# draw
g + a1
g + a2
g + a1 + a2

But in the case of g + a1 + a2 I obtain the first picture with only the first plot p1 inserted. What's wrong? How to draw two and more plots using annotation_custom()?

It's a weird bug that I noticed recently; for some grobs that ggplot2 considers similar, the position can be ignored and they end up superposed:

myGrob <- rectGrob(gp=gpar(fill="red", alpha=0.5), name="whatever")
myGrob2 <- rectGrob(gp=gpar(fill="blue", alpha=0.5))

# this is fine
qplot(1:10, 1:10) + theme(plot.margin=unit(c(0, 3, 0, 0), "cm")) +
  annotation_custom(myGrob, xmin=5, xmax=6, ymin=3.5, ymax=5.5) +
  annotation_custom(myGrob2, xmin=8, xmax=12, ymin=3.5, ymax=5.5) 

# using twice the same grob, they just sit on top of each other
p <- qplot(1:10, 1:10) + theme(plot.margin=unit(c(0, 3, 0, 0), "cm")) +
  annotation_custom(myGrob, xmin=5, xmax=6, ymin=3.5, ymax=5.5) +
  annotation_custom(myGrob, xmin=8, xmax=12, ymin=3.5, ymax=5.5) 

g <- ggplotGrob(p)
grid.ls(g$grobs[[4]]) # two of them

I have no idea what's causing this, but presumably it's related to your problem.