[R] Missing shapes in legend with scale_shape_manual

[Kevin Zembower]

Hello,

I'm trying to plot a graph of blood glucose versus date. I also record
conditions, such as missing the previous night's medications, and
missing exercise on the previous day. My data looks like:

> b2[68:74,]
# A tibble: 7 × 5
  Date       Time      bg missed_meds no_exercise
  <date>     <time> <dbl> <lgl>       <lgl>      
1 2023-10-17 08:50    128 TRUE        FALSE      
2 2023-10-16 06:58    144 FALSE       FALSE      
3 2023-10-15 09:17    137 FALSE       TRUE       
4 2023-10-14 09:04    115 FALSE       FALSE      
5 2023-10-13 08:44    136 FALSE       TRUE       
6 2023-10-12 08:55    122 FALSE       TRUE       
7 2023-10-11 07:55    150 TRUE        TRUE       
> 

This gets me most of the way to what I want:

ggplot(data = b2, aes(x = Date, y = bg)) +
    geom_line() +
    geom_point(data = filter(b2, missed_meds),
               shape = 20,
               size = 3) +
    geom_point(data = filter(b2, no_exercise),
               shape = 4,
               size = 3) +
    geom_point(aes(x = Date, y = bg, shape = missed_meds),
               alpha = 0) + #Invisible point layer for shape mapping
    scale_y_continuous(name = "Blood glucose (mg/dL)",
                       breaks = seq(100, 230, by = 20)
                       ) +
    geom_hline(yintercept = 130) +
    scale_shape_manual(name = "Conditions",
                       labels = c("Missed meds",
                                  "Missed exercise"),
                       values = c(20, 4),
                       ## size = 3
                       )

However, the legend just prints an empty square in front of the labels.
What I want is a filled circle (shape 20) in front of "Missed meds" and
a filled circle (shape 4) in front of "Missed exercise."

My questions are:
 1. How can I fix my plot to show the shapes in the legend?
 2. Can my overall plotting method be improved? Would you do it this
way?

Thanks so much for your advice and guidance.

-Kevin