Improving plot readability

Question

I need to look for correlations in the publicly available flights package. I managed to make a scatter plot using ggplot. With the code:

library(nycflights13)
attach(flights)
ggplot(flights, aes(x = arr_delay, y = dep_delay)) + 
  geom_point(size = 2) +
  geom_smooth(method="auto", se=TRUE, fullrange=FALSE, level=0.95)

As show in the image most is centered in the bottom left. Is there any way to make this graph look more visually appealing by spreading the plotted values better?

as an aside: [Why is it not advisable to use `attach()` in R, and what should I use instead?](https://stackoverflow.com/questions/10067680/why-is-it-not-advisable-to-use-attach-in-r-and-what-should-i-use-instead) — markus, Mar 07 '20 at 11:05
;) the inevitable [attach bashing](https://www.r-bloggers.com/to-attach-or-not-attach-that-is-the-question/) beginneth ... even more so since the example works totally fine without the `attach` line ;) - But on a serious note regarding the actual question: Not sure if it would really **improve** readability, but you could use `ggplot2::scale_x_continuous` `trans` argument to use logarithmic scales for x and y axis... i.e. `+ scale_x_continuous(trans='log10')` — dario, Mar 07 '20 at 11:09
@dario Yeah, someone had to bring up that post ;). Regarding the question, the data contains negative values so log won't work. — markus, Mar 07 '20 at 11:12
@markus Thanks for the comment, I didn't even check if there are values <= 0 in the data.. log transform won't work for obvious reasons ;) I think your [`geom_hex`](https://ggplot2.tidyverse.org/reference/geom_hex.html) link is broken otherwise interesting and good suggestion — dario, Mar 07 '20 at 11:33
Since the variables are called "_delay", you could argue that any negative values should be omitted. Then you can take logs (after adding a small number first). As another aside, always label your axes correctly. What are the units? Minutes? Hours? Days? — Edward, Mar 07 '20 at 11:43

score 3 · Answer 1 · answered Mar 07 '20 at 11:09

You can plot your points by using the alpha parameter which gives a degree of transparency (between 0 and 1 being the most opaque) to them. This will make overlapping points distinguish better while also making the regions of the plot with higher concentration look darker. The style of the plot will improve, too.

Start with a value of alpha = 0.7 then experiment with it until you get the best results.

ggplot(flights, aes(x = arr_delay, y = dep_delay)) + 
  geom_point(size = 2, alpha = 0.7) +
  geom_smooth(method="auto", se=TRUE, fullrange=FALSE, level=0.95)

score 3 · Answer 2 · answered Mar 07 '20 at 13:22

I used facets to separate the flights that arrived early or on time (arr_delay <=0) with those that arrived late (arr_delay>0). The relationship seems different.

library(nycflights13)
library(dplyr)
library(ggplot2)

ff <- flights %>%
  filter(!is.na(arr_delay), origin=="LGA") %>%  # Filtered to reduce waiting time!
  mutate(`Arrival time`=ifelse(arr_delay<=0, "Early", "Delayed"))

ggplot(ff, aes(x = arr_delay, y = dep_delay)) + 
       geom_point(size = 2, alpha = 0.3) +
       geom_smooth(method="auto", fullrange=FALSE, level=0.95) + 
       facet_wrap(~`Arrival time`, scales="free", labeller=label_both) +
       labs(x="Arrival delay (minutes)", y="Departure delay (minutes)")

So this basically zooms in on a subset of the data (I am not familiar with facet_wrap())? — SomeDutchGuy, Mar 07 '20 at 13:29
It zooms in on the flights that arrived early since facetting allows the axis scales to vary or be "free". — Edward, Mar 07 '20 at 13:36

score 1 · Accepted Answer · answered Mar 07 '20 at 11:40

For the points, you could use aggregated data, for the smooth the normal data.

flights <- within(flights, {
  bin <- floor(dep_delay / 10)
  av_arr <- ave(arr_delay, bin, FUN=mean)
  av_dep <- ave(dep_delay, bin, FUN=mean)
})

library("ggplot2")
library("nycflights13")
ggplot(flights) + 
  geom_point(aes(x=av_arr, y=av_dep), size=2) +
  geom_smooth(aes(x=arr_delay, y=dep_delay), method="auto", se=TRUE, 
              fullrange=FALSE, level=0.95)

Improving plot readability

3 Answers3