The chronological chart
This example highlights the structure of a chronological dataset,
called chrons, with a x-axis included for educational
reasons:
chrons <- add_chron(
region = c("region = A", "region = A", "region = A", "region = A", "region = A", "region = A", "region = A", "region = A", "region = A", "region = A", "region = A", "region = A", "region = B", "region = B", "region = B"),
name = c("level = 1\nadd =\nFALSE", "level = 2\nadd =\nFALSE", "level = 3\nadd =\nFALSE", "level = 4\nadd =\nFALSE", "level = 5\nadd =\nFALSE","level = 1\nadd =\nTRUE","level = 2\nadd =\nTRUE","level = 2\nadd =\nTRUE", "add =\nTRUE", "level = 3", "add = TRUE", "level = 4", "level = 1\nadd = FALSE", "level = 2\nadd = FALSE", "level = 3\nadd = FALSE"),
start = c(-500, -500, -500, -500, -500, -400, -400, 0, 0, "200/200", "200/200", "275_325", -500, -500, -500),
end = c(500, 500, 500, 500, 500, 400, -50, 400, "200/200", 400, "275_325", 400, 500, 500, 500),
level = c(1, 2, 3, 4, 5, 1, 2, 2, 3, 3, 4, 4, 1, 2, 3),
add = c(FALSE, FALSE, FALSE, FALSE, FALSE, TRUE, TRUE, TRUE, TRUE, TRUE, TRUE, TRUE, FALSE, FALSE, FALSE),
new_table = TRUE)
# How does it look like?
print(chrons)
#> # A tibble: 15 x 6
#> region name start end level add
#> <chr> <chr> <chr> <chr> <dbl> <lgl>
#> 1 region = A "level = 1\nadd =\nFALSE" -500 500 1 FALSE
#> 2 region = A "level = 2\nadd =\nFALSE" -500 500 2 FALSE
#> 3 region = A "level = 3\nadd =\nFALSE" -500 500 3 FALSE
#> 4 region = A "level = 4\nadd =\nFALSE" -500 500 4 FALSE
#> 5 region = A "level = 5\nadd =\nFALSE" -500 500 5 FALSE
#> 6 region = A "level = 1\nadd =\nTRUE" -400 400 1 TRUE
#> 7 region = A "level = 2\nadd =\nTRUE" -400 -50 2 TRUE
#> 8 region = A "level = 2\nadd =\nTRUE" 0 400 2 TRUE
#> 9 region = A "add =\nTRUE" 0 200/200 3 TRUE
#> 10 region = A "level = 3" 200/200 400 3 TRUE
#> 11 region = A "add = TRUE" 200/200 275_325 4 TRUE
#> 12 region = A "level = 4" 275_325 400 4 TRUE
#> 13 region = B "level = 1\nadd = FALSE" -500 500 1 FALSE
#> 14 region = B "level = 2\nadd = FALSE" -500 500 2 FALSE
#> 15 region = B "level = 3\nadd = FALSE" -500 500 3 FALSE
Admittedly, the names of the chrons are not
pretty but they are intended to show that the information of each
chron is directly mirrored in the plot and can be reconstructed
from it.
How does it work?
Each region is plotted independently. Within a region, all
chrons with add = FALSE will be evenly scaled
according to the maximum level, e.g. in region A in the
add = FALSE-column the maximum level is 5,
hence, all chrons will be 1/5 wide. Similarly in region B, the
maximum levelis 3, consequently all chrons are 1/3
wide. This essentially transform the x-axis into percentages, allowing
to easily add additional labels or customise the position and angle of
the chrons’ names. Likewise, all chrons in the
add = TRUE-column are displayed with their x-value shifted
by 1, i.e. they are plotted between 1 and 2.
If not all chrons are further subdivided, empty space is filled
by the rightmost chrons. However, by default the names of
chrons from the same level will be placed on the same x-value.
In complex charts this increases readability and ensures appropriate
space for additional labels, even if it can produce odd looking empty
space in small plots (see above).
Unclear start and end dates
Unclear start and end dates can be
displayed in two ways: as dashed horizontal lines or as solid diagonal
lines. The dashed horizontal lines between two chrons appears
when the respective start and/or end date of a
chron are coded as "300/300", indicating that they
cannot be stated precisely. The diagonal lines are coded with as
"300_350" and indicate a transition of a known duration
between two periods. The reasons for unclear or vague transitions are
manifold. To name a few: the transition between two chronological units
or strata are blurred, sites within a region yield different dates for
the same transition, transitions are not always clearly expressed in the
material record, or it is debated whether this transition exists.
To indicate a transition period, its start and
end date are given, e.g. "50/100" or
"50_100". For a distinct date, the both years are the same.
For example, "100/100" would result in a single horizontal
dashed line.
Does the order of the dates matter? Lets play around by using a
different region for the different combinations (using
arrange_regions() to modify their default alphabetical
order in the plot):
data <- add_chron(region = "earlier/later",
name = c("1", "2", "1a", "1b"),
start = c(-100, "50/100", -100, "-25_25"),
end = c("50/100", 200, "-25_25", "50/100"),
level = c(1, 1, 2, 2),
add = FALSE,
new_table = TRUE) %>%
add_chron(region = "later/earlier",
name = c("1", "2", "1a", "1b"),
start = c(-100, "100/50", -100, "25_-25"),
end = c("100/50", 200, "25_-25", "100/50"),
level = c(1, 1, 2, 2),
add = FALSE,
new_table = FALSE) %>%
add_chron(region = "mixed",
name = c("1", "2", "1a", "1b"),
start = c(-100, "50/100", -100, "-25_25"),
end = c("50/100", 200, "25_-25", "100/50"),
level = c(1, 1, 2, 2),
add = FALSE,
new_table = FALSE) %>%
add_chron(region = "same",
name = c("1", "2", "1a", "1b"),
start = c(-100, "100/100", -100, "25_25"),
end = c("100/100", 200, "25_25", "100/100"),
level = c(1, 1, 2, 2),
add = FALSE,
new_table = FALSE) %>%
arrange_regions(order = c("earlier/later", "later/earlier", "same", "mixed"))
plot_chronochrt(data)
Their order matters: Diagonal lines are always drawn from the first to the second value and changing the order will result in a different orientation. Dashed lines do not depend on the order that much. However, consistency matters because mixed orders can result in a failure to recognize the true end of the period. Additionally, the vertical position of the labels are based on the first value.
