ggplot：向散点图添加线条轮廓

Question

I have thousands of datapoints across 10 minute time stamps over several days. Drawing a scatterplot creates a repeating pattern, which I would like to emphasize. In the image above, I would like to have a line drawn, outlining the shape of this scatter. More specifically, a line going through the maximum value of value for each hour tick. I've tried adding the likes of freqpoly and hist but those do not fit this plot type (where x is the timestamps). I've also tried calculating the maximum per timestamp, but I can't use this in the same pot because the original data is in tall format (with multiple entries for every time stamp).

Example data:

set.seed(999)
df <- data.frame('hour' = rep(seq(ISOdatetime(2019,12,1,0,0,0), by = '10 mins', length.out = 6), 3),
                 'value' = rnorm(18),
                 'category' = rep(c('a', 'b', 'c'), 6))

ggplot(df, aes(x = hour, y = value)) +
  geom_point(aes(color = category), cex = 7) +
  theme_minimal()

And this is what I'd like the final product to look like ( black line added by hand ):

Answer 1

You can do this by moving the aesthetics for the geom_point into the actual argument. Then you can add a stat_summary to add the line as follows:

set.seed(999)
library(ggplot2)

df <- data.frame('hour' = rep(seq(ISOdatetime(2019,12,1,0,0,0), by = '10 mins', length.out = 6), 3),
                 'value' = rnorm(18),
                 'category' = rep(c('a', 'b', 'c'), 6))

# Valid for ggplot2 version 3.2.1.9000
# fun.y might be needed if running an earlier version
ggplot(df) +
  geom_point(aes(x = hour, y = value, color = category), cex = 7) +
  theme_minimal()+
  stat_summary(geom = "line", fun = max, aes(hour, value))

# Or you can simplify a little and just keep the color aesthetic in the geom_point
# Same result achieved

ggplot(df, aes(x = hour, y = value),) +
  geom_point( aes(color = category), cex = 7) +
  stat_summary(geom = "line", fun = max)+
  theme_minimal()

This allows you to add the lines and then add the summary statistics as a new "line" geometry.