ggplot2: Inconsistent color from alpha - r

Question

I am making several plots that have different x-axis limits, and I want to highlight a region of interest by adding a grey box. Even though I use the same geom_rect() command with the same alpha value in ggplot2, I get results with very different grey colors. I have looked here and here but so far have not figured out how to make these boxes the same level of transparency. Below is a reproducible example (with fake data) and the figures that it produces. Notice the different color of the grey rectangles. I want the grey to be the same across plots.
Data<-structure(list(X = c(34L, 27L, 28L, 47L, 26L, 3L, 13L, 31L, 39L,
16L, 45L, 5L, 49L, 17L, 29L, 43L, 1L, 35L, 41L, 10L, 48L, 24L,
12L, 11L, 30L, 40L, 8L, 4L, 20L, 25L, 50L, 22L, 9L, 21L, 18L,
7L, 15L, 44L, 6L, 36L, 46L, 33L, 2L, 37L, 23L, 14L, 42L, 38L,
19L, 32L, 34L, 27L, 28L, 47L, 26L, 3L, 13L, 31L, 39L, 16L, 45L,
5L, 49L, 17L, 29L, 43L, 1L, 35L, 41L, 10L, 48L, 24L, 12L, 11L,
30L, 40L, 8L, 4L, 20L, 25L, 50L, 22L, 9L, 21L, 18L, 7L, 15L,
44L, 6L, 36L, 46L, 33L, 2L, 37L, 23L, 14L, 42L, 38L, 19L, 32L
), Y = c(130L, 146L, 58L, 110L, 117L, 135L, 133L, 108L, 97L,
61L, 71L, 64L, 103L, 142L, 125L, 104L, 100L, 147L, 111L, 78L,
56L, 145L, 62L, 69L, 70L, 116L, 137L, 79L, 150L, 94L, 91L, 81L,
65L, 118L, 129L, 83L, 98L, 84L, 85L, 148L, 93L, 73L, 59L, 87L,
134L, 88L, 136L, 90L, 140L, 55L, 89L, 115L, 123L, 51L, 132L,
126L, 66L, 80L, 60L, 120L, 109L, 76L, 74L, 57L, 149L, 121L, 138L,
128L, 114L, 127L, 68L, 107L, 67L, 112L, 144L, 119L, 53L, 52L,
54L, 96L, 131L, 106L, 113L, 72L, 95L, 63L, 92L, 86L, 75L, 105L,
82L, 101L, 139L, 143L, 122L, 77L, 99L, 141L, 124L, 102L), B = structure(c(2L,
2L, 1L, 1L, 2L, 2L, 1L, 1L, 1L, 1L, 2L, 2L, 1L, 1L, 2L, 2L, 1L,
1L, 1L, 1L, 2L, 2L, 1L, 1L, 2L, 2L, 1L, 1L, 1L, 1L, 2L, 2L, 1L,
1L, 2L, 2L, 1L, 1L, 1L, 1L, 2L, 2L, 1L, 1L, 2L, 2L, 1L, 1L, 1L,
1L, 2L, 2L, 1L, 1L, 2L, 2L, 1L, 1L, 1L, 1L, 2L, 2L, 1L, 1L, 2L,
2L, 1L, 1L, 1L, 1L, 2L, 2L, 1L, 1L, 2L, 2L, 1L, 1L, 1L, 1L, 2L,
2L, 1L, 1L, 2L, 2L, 1L, 1L, 1L, 1L, 2L, 2L, 1L, 1L, 2L, 2L, 1L,
1L, 1L, 1L), class = "factor", .Label = c("no", "yes"))), .Names = c("X",
"Y", "B"), row.names = c(NA, -100L), class = "data.frame")
Data2<-structure(list(variable = c(2.49676547444708, 0.67359598601097,
0.674751772966082, 0.0317590441796792, 0.485143583939748, 1.08231639527806,
0.0732344181040914, 1.62357048819912, 0.146833215667032, 0.823157103468943,
0.240761579418538, 1.37540376416553), DOY_mid_month = c(15, 46,
75, 106, 136, 167, 197, 228, 259, 289, 320, 350)), .Names = c("variable",
"DOY_mid_month"), row.names = c(NA, -12L), class = "data.frame")
test<-ggplot(data=Data) +
geom_rect(aes(xmin=5, xmax=30, ymin=1, ymax=40), alpha = 0.02) +
geom_point(aes(x = X, y = X, colour= B), data =Data, size=2) +
theme_bw()
test2 <-ggplot(data=Data2) +
geom_rect(aes(xmin=5, xmax=30, ymin=-Inf, ymax=Inf), alpha = 0.02) +
geom_point(aes(x = DOY_mid_month, y = variable), color="black", size=4) +
scale_x_continuous("Day of Year", limits = c(0, 366)) + # Use this to add back X-axis label for the bottom plot in panel
scale_y_continuous(expression(paste("Variable", sep=""))) +
theme_bw()
Plot result from first example:
Plot result from second example:

Answer 1

You are currently drawing one rectangle for each row of the dataset. The more rectangles you overlap, the darker they get, which is why the longer dataset has a darker rectangle. Use annotate instead of geom_rect to draw a single rectangle.
annotate(geom = "rect", xmin=5, xmax=30, ymin=-Inf, ymax=Inf, alpha = 0.2)
If you want to stick with geom_rect you can give a one row data.frame to that layer so that each rectangle is only drawn one time. Here I use a fake dataset, although you could put your rectangle limits in the data.frame, as well.
geom_rect(data = data.frame(fake = 1),
aes(xmin = 5, xmax= 30, ymin = -Inf, ymax = Inf), alpha = 0.2)