overlapping balloon plots in ggplot

Question

I would like to overlap these two plots into one:

But I would like to have the largest point in the background with the smallest one in the foreground regardless of plant type.

I would basically like to do something like this:

But the only problem is that the plants are substantially different on size so I would like to keep their density point size individual (keep their individual ranges).

If I can't keep their own size range, but make it so the largest point is behind that will be less ideal but workable with what I want to do.

EDIT: Here is some example data:

TRANSECT FRAME PLANT DENSITY
1        1      SCAR    3600
1        2      SCAR    3840
1        3      SCAR    0
1        4      SCAR    1880
2        1      SCAR    3480
2        2      SCAR    1600
2        3      SCAR    3640
2        4      SCAR    1280
1        1      BRRU    40
1        2      BRRU    40
1        3      BRRU    1320
1        4      BRRU    280
2        1      BRRU    120
2        2      BRRU    0
2        3      BRRU    120
2        4      BRRU    360

The densities are on two different scales as one is a very small skinny plant and the other is taller and takes up more space.

Thanks!

Answer 1

(BTW, it is good practice to include a reproducible example so that people who want to help you don't have to do extra work that you've already done, or make guesses about your data that might hinder you getting an appropriate answer.)

Here I've made some fake data and graphed the way your example looks:

library(dplyr); library(ggplot2)
set.seed(42)
df <- data.frame(FRAME = rep(1:12, each = 4),
                 TRANSECT = rep(1:4, 12),
                 BRRU = runif(48, 0, 3000),
                 SCAR = runif(48, 0, 3000)) %>%
  tidyr::gather(PLANT, DENSITY, BRRU:SCAR)
ggplot(df, aes(FRAME, TRANSECT, size = DENSITY, color = PLANT)) + 
  geom_point()

By grouping by each coordinate and sorting in descending order, the largest points print first, with the smaller ones printing afterwards, on top:

ggplot(df %>% 
         group_by(FRAME, TRANSECT) %>% 
         arrange(-DENSITY) %>%
         ungroup(), 
       aes(FRAME, TRANSECT, size = DENSITY, color = PLANT)) + 
  geom_point()