How to fill the area under/above the curve with different colors above and below 0?

Question

I would like to plot a time series of the Standardized Precipitation Index (SPI). Normally this looks somewhat like this:

You can see that the area under/above the curve is colored in blue/red. This is what I would like to plot too.

I know that there were kind of similar questions, like this and that one. This could bring be a bit further, but unfortunately not to the final result yet.

To make it easier to understand, here is some code so that everybody can reproduce it:

library(data.table)
library(ggplot2)

vec1 <- 1:310
vec2 <- c(1.78, 1.88,  1.10,  0.42,  0.73,  1.35,  1.34,  0.54,  0.20,  0.72,  1.29,  1.78,  1.30,  1.37, -0.13,  0.64, -0.13,  0.87,  0.47, -0.26, -0.27,
          -0.81, -0.54, -0.77, -0.29, -0.22, -0.05,  0.41,  0.45,  0.91, -0.31,  0.67,  0.28,  0.93,  0.43, -0.04, -0.80, -1.20, -0.73, -0.98,  0.47, -0.01,
          1.30,  1.45,  0.72, -0.59, -1.14, -0.33,  0.22,  0.49,  0.58,  0.36,  0.66,  0.64,  0.47, -0.60,  1.01,  1.50,  1.18,  0.82,  0.02,  0.57,  0.25, 
          1.20,  1.19,  0.71, -0.30, -1.37, -1.50, -1.03, -0.77, -1.08, -1.92, -2.32, -2.46, -1.61, -0.39,  0.67,  0.38,  0.62, -0.34,  0.01, -0.55, -0.74,
          -1.95, -1.18, -0.96,  0.36, -0.96, -1.28, -2.29, -2.67, -0.65, -0.13,  0.61,  0.21,  0.57,  0.11,  0.37,  0.20, -0.14, -0.87, -0.84,  0.87,  1.33,
          0.45, -0.76, -1.27, -0.65, -0.29,  0.54,  0.14, -0.55, -0.94, -0.98, -0.44, -0.37,  0.72,  0.70,  0.95,  0.89,  1.10,  1.51,  1.11,  1.77,  1.20,
          1.23, -0.72, -1.43, -2.11, -1.37, -0.80, -0.34, -0.14,  0.22, -0.65, -0.44, -0.86, -0.46, -0.67, -0.91, -0.40, -0.09,  0.22,  0.96,  0.71,  0.51,
          -1.61, -1.62, -1.43, -0.27,  1.08,  1.76,  1.30,  0.78,  1.02,  1.01,  0.56, -0.32,  0.37,  0.31,  1.36,  1.49,  1.42,  0.78, -0.19,  0.64,  0.39,
          0.47, -1.13, -1.45, -0.52,  0.43, -0.19, -0.97, -0.27,  0.63,  1.01,  1.01,  0.83, -0.56, -1.71, -0.29,  1.06,  1.82,  1.28,  0.88,  1.08,  1.78,
          1.47,  0.74, -0.34,  0.14,  1.09,  1.49,  1.30,  0.28, -0.25,  0.24, -0.33,  0.05, -0.86, -0.69, -1.03, -0.59,  0.32,  0.61,  0.84, -0.18, -0.67,
          0.46,  0.31, -0.72, -2.26, -2.85, -0.69, -0.77,  0.64, -1.49, -1.69, -1.55, -0.28, -0.80, -1.15, -0.38,  0.31,  0.18, -0.27, -0.84, -0.94, -1.23,
          -0.53, -1.52, -0.73, -0.93,  0.25, -0.11,  0.38,  0.48,  0.10, -0.02,  0.26,  1.39,  1.61,  0.83,  0.09,  0.95,  1.07,  0.77,  0.23,  0.26,  0.85,
          0.93,  0.91,  1.10,  0.47,  0.74,  1.42,  1.17,  0.32, -0.40,  0.76,  1.44,  1.69,  1.03,  0.01,  0.46,  0.61,  0.60, -0.09, -0.31, -0.96, -0.91,
          -0.06,  0.75,  1.32,  1.29,  0.55,  0.43, -1.25,  0.12, -0.05,  0.18, -0.77, -2.19, -1.85, -2.12, -1.51, -1.14, -0.79, -0.82, -1.13, -1.72, -2.14,
          -1.95, -0.63,  0.70,  0.64,  0.17, -1.04, -0.58, -0.57, -0.57, -1.05, -1.11, -0.59, -0.07,  1.22,  0.30, -0.15)

df <- data.frame(vec1, vec2)
colnames(df) <- c("ID", "SPI")

df = as.data.table(df)

There you have the entire SPI time series, one value for each month between 1980 and 2005.

So far I came to that result:

ggplot(data = df, aes(x = ID, y = SPI)) +
  geom_col(data = df[SPI <= 0], fill = "red") +
  geom_col(data = df[SPI >= 0], fill = "blue") +
  theme_bw()

When you run the code you will see the following plot:

This is going into the right direction, but the small white gaps between values in the first half are disturbing and not supposed to be there. So there must be something wrong.

It is supposed to look like this, just with the two colors blue and red for the positive and negative values:

ggplot(data = df, aes(x = ID, y = SPI)) +
  geom_area() +
  theme_bw()

The code leads to this image:

You can see that there are no white gaps between the single values and I have absolutely no idea what leads to these errors.

Anybody with an idea how to solve that?

score 2 · Accepted Answer · answered Mar 26 '21 at 16:49

OP, what you are observing are the artifacts related to the resolution of your graphics driver and the space between columns. The areas you show are composed of many filled columns next to one another on the x axis. You do not specify the width= argument for geom_col(), so the default value leaves a space between the individual values on the x axis. It's best to illustrate if we take only a section of your data along the x axis:

ggplot(data = df, aes(x = ID, y = SPI)) +
  geom_col(data = df[SPI <= 0], fill = "red") +
  geom_col(data = df[SPI >= 0], fill = "blue") +
  theme_bw() +
  xlim(0,100)  # just the first part on the left

There's your white lines - it's the space bewtween the columns. When you have the larger picture, the appearance of the white lines has to do with the resolution of your graphics device. You can test this if you save your graphic with ggsave() using different parameters for dpi=. For example, on my computer saving ggsave('filename.png', dpi=72) gives no lines, but ggsave('filename.png', dpi=600) shows the white lines in places.

There's an easy solution to this though, which is to specify the width= argument of geom_col() to be 1. Be default, it's set to 0.75 or 0.8 (not exactly sure), which leaves a gap between the next value (fills ~75 or 80% of the space). If you set this to 1, it fills 100% of the space allotted for that column, leaving no white space in-between:

ggplot(data = df, aes(x = ID, y = SPI)) +
  geom_col(data = df[SPI <= 0], fill = "red", width=1) +
  geom_col(data = df[SPI >= 0], fill = "blue", width=1) +
  theme_bw() +
  xlim(0,100)

thanks a lot for the very quick answer. Now the result looks very good, almost exactly what I wanted! I applied this code now: ggplot(data = df, aes(x = ID, y = SPI)) + geom_col(data = df[SPI <= 0], fill = "red", width=1) + geom_col(data = df[SPI >= 0], fill = "blue", width=1) + theme_bw() The only thing that is left now is that it looks a little bit too much like single colums compared to the first and third picture I showed above. Is there a way to change that, to make it kind of smoother? — climsaver, Mar 26 '21 at 16:57
You would have to model it with `geom_area` and then apply a different `fill` aesthetic based on the value of the y axis. It will not be as straightforward as it would seem, since there is not always a y=0 point whenever the sign changes. You can also "fake" it a bit by interpolating the value in-between each of the points in your dataset. You can even do that a few times to make the path smoother between them, then still model the data using `geom_col()`. — chemdork123, Mar 26 '21 at 18:31
to be honest that sounds quite complicated and at the moment I would not have an idea of how to do that. — climsaver, Mar 29 '21 at 07:58

score 0 · Answer 2 · answered Mar 26 '21 at 17:03

0

What if you change the distance between columns? Just added "width=1" inside "aes" of ggplot.

ggplot(data = df, aes(x = ID, y = SPI, width=1)) +
  geom_col(data = df[SPI <= 0], fill = "red") +
  geom_col(data = df[SPI >= 0], fill = "blue") +
  theme_bw()

I get this:

image.result

answered Mar 26 '21 at 17:03

Nikolas Schiozer

19
1

Thanks for that hint. It leads basically to the same result as the answer of chemdork123. The Code is just a bit shorter this way. – climsaver Mar 29 '21 at 07:57

How to fill the area under/above the curve with different colors above and below 0?

2 Answers2