Alpha aesthetic doesn't work with geom_segment - why?

Question

I've been playing around with some football data and came across the problem with alpha aesthetic use. Basically I tried to plot some passes on the football pitch created as here: https://github.com/FCrSTATS/Visualisations/blob/master/3.CreateAPitch.md However, while doing so with geom_segment and alpha aesthetic to I ended up without the lines when alpha was less than 1, only the arrows were plotted. What's the reason? Is it something with theme defined in a function from the link mentioned before? A code I have used:

pitch <- createPitch(10600, 7040, "#202020", "#797876", "#202020", "#131313")

pitch + geom_segment(data = data,
                 aes(x = x/100 * xmax, y = y/100 * ymax, 
                     xend = as.numeric(as.character(pass_end_x))/100 * xmax, 
                     yend = as.numeric(as.character(pass_end_y))/100 * ymax,
                     alpha = data$n/max(data$n)),
                 color = "blue", 
                 arrow = arrow(length = unit(0.3, "cm"), ends = 'first', type = 'closed'))

and the data:

structure(list(x = c(77.076, 66.7944444444444, 60.425, 64.7828571428571
), y = c(77.672, 19.0888888888889, 52.8055555555556, 90.0485714285714
), pass_end_x = c(88.296, 77.8240740740741, 70.0472222222222, 
73.5857142857143), pass_end_y = c(61.204, 20.7796296296296, 
64.0083333333333, 89.8485714285714), n = c(25L, 54L, 36L, 35L)), .Names = 
c("x", "y", "pass_end_x", "pass_end_y", "n"), row.names = c(NA, -4L), class 
= c("tbl_df", "tbl", "data.frame"))

Never use `$` inside `aes()`. If you just put `alpha = n / max(n)` in your mapping it will probably work. — Gregor Thomas, Mar 10 '18 at 18:24

Gregor Thomas · Answer 1 · 2018-03-10T19:53:04.053

Ignoring the pitch part, getting rid of the $ works just fine:

dd = structure(list(x = c(77.076, 66.7944444444444, 60.425, 64.7828571428571
), y = c(77.672, 19.0888888888889, 52.8055555555556, 90.0485714285714
), pass_end_x = c(88.296, 77.8240740740741, 70.0472222222222, 
73.5857142857143), pass_end_y = c(61.204, 20.7796296296296, 
64.0083333333333, 89.8485714285714), n = c(25L, 54L, 36L, 35L)), .Names = 
c("x", "y", "pass_end_x", "pass_end_y", "n"), row.names = c(NA, -4L), class 
= c("tbl_df", "tbl", "data.frame"))

xmax = ymax = 1

p = ggplot(
    dd,
    aes(
        x = x / 100 * xmax,
        y = y / 100 * ymax,
        xend = as.numeric(as.character(pass_end_x)) / 100 * xmax,
        yend = as.numeric(as.character(pass_end_y)) / 100 * ymax,
        alpha = n / max(n)
    )
) +
    geom_segment(color = "blue",
                             arrow = arrow(
                                length = unit(0.3, "cm"),
                                ends = 'first',
                                type = 'closed'
                             ))

p

You can easily see the transparency differences. Adding the theme from your link is still fine:

p + theme_blankPitch()

Adding the pitch from your question makes the lines harder to see due to your color choices, but they are still there:

pitch <- createPitch(10600, 7040, "#202020", "#797876", "#202020", "#131313")
pitch + geom_segment(data = dd,
    aes(
        x = x / 100 * xmax,
        y = y / 100 * ymax,
        xend = as.numeric(as.character(pass_end_x)) / 100 * xmax,
        yend = as.numeric(as.character(pass_end_y)) / 100 * ymax,
        alpha = n / max(n)
    ), color = "blue",
                             arrow = arrow(
                                length = unit(0.3, "cm"),
                                ends = 'first',
                                type = 'closed'
                             ))

If you increase the size of the segments (say, size = 1.5) they are a little clearer, but I would recommend different color choices. Another adjustment you can make is to set the limits of scale_alpha_continuous to make the minimum of the range more opaque. The default minimum is 0.1, you could raise it to 0.2 or even higher.

Well, this is quite embarrassing, but what I have noticed now is that the lines were not displayed in the plot pane. After zooming out there were displayed ok. Anyway, thanks for the help! — jakes, Mar 10 '18 at 20:11

Alpha aesthetic doesn't work with geom_segment - why?

1 Answers1