ggplot2 faceted line plot has areas of the line filled with solid color, why?

ggplot2, r

Solution

Note the difference between `geom_line` and `geom_path` applied to the same data.

library(ggplot2)

x = c(seq(1, 10, 1), seq(10, 1, -1))
y = seq(0, 19, 1)
df = data.frame(x, y)

ggplot(df, aes(x, y)) + geom_line()
ggplot(df, aes(x, y)) + geom_path()

Note the order in the `df` data frame.

    x  y
1   1  0
2   2  1
3   3  2
4   4  3
5   5  4
6   6  5
7   7  6
8   8  7
9   9  8
10 10  9
11 10 10
12  9 11
13  8 12
14  7 13
15  6 14
16  5 15
17  4 16
18  3 17
19  2 18
20  1 19

`geom_path` plots in order of the observations.

`geom_line` plots in order of x values.

The effect is more marked when the x values are closer together.

x = c(seq(1, 10, .01), seq(10, 1, -.01))
y = seq(.99, 19, .01)
df = data.frame(x, y)

ggplot(df, aes(x, y)) + geom_line() 
ggplot(df, aes(x, y)) + geom_path()

Problem

I have a question regarding a weird result for a line plot that uses facets. I have water data masurements for different depth (=pressures). The data comes as a table as such: ``` Pressure Temperature pH 0 30 8.1 1 28 8.0 ``` I "melt" this data to yield: ``` Pressure variable value 0 Temperature 30 1 Temperature 30 0 pH 8.1 1 pH 8.0 ``` and so on. I now plot this: ``` ggplot(data.m.df, aes(x=value, y=Pressure)) + facet_grid(.~variable, scale = "free") + scale_y_reverse() + geom_line() + opts(axis.title.x=theme_blank()) ``` It kinda works, except there are parts of the line plot that get filled with solid color. I have no idea why, especially because it works just fine if I exchange x for y and use "variable ~ ." as the facet_grid formula.

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