Plotting 3 Variables on One Chart - ggplot - r

Question

I have some experience with base R but am trying to learn tidyverse and ggplot. I have a dataframe with 4 columns of data. I want a simple x-y plot, where the first column of data is on the x-axis, and the data in the other 3 columns is plotted on the y-axis, resulting in 3 lines on one plot. The first 15 lines of my data look like this (sorry about the image - I don't know how to insert a sample of my data):
screen shot - first 15 rows of data
I tried to plot the second and third columns of data as follows: ,
ggplot(data=SWRC_SL, aes(x=SWRC_SL$pressure_head, y=SWRC_SL$UNSODA_theta)) +
geom_line(colour="red") + scale_x_log10() +
ggplot(data=SWRC_SL, aes(x=SWRC_SL$pressure_head, y=SWRC_SL$Vrugt_theta)) +
geom_line(colour="blue") + scale_x_log10()
I get this error:
Error: Don't know how to add ggplot(data = SWRC_SL, aes(x = SWRC_SL$pressure_head, y = SWRC_SL$Vrugt_theta)) to a plot
I believe I should be using something like "group=" to indicate which columns should be plotted, but I haven't been able to find an example that shows how you can use gglot to plot data across multiple columns. What am I missing ?

Answer 1

ggplot() is only ever called once when you create a chart. Try with the following:
ggplot() +
geom_line(data=SWRC_SL, aes(x=pressure_head, y=UNSODA_theta), colour="red") +
geom_line(data=SWRC_SL, aes(x=pressure_head, y=Vrugt_theta), colour="blue") +
scale_x_log10()
A better method would be to turn your data to long, where the UNSODA_theta and Vrugt_theta data are in the same column (say thetas), and have another column (say type_theta) indicating whether the data is for UNSODA_theta or Vrugt_theta. Then you could do the following:
ggplot(data=SWRC_SL, aes(x=pressure_head, y=thetas, colour=type_theta)) +
geom_line() +
scale_x_log10()
This is more desirable because ggplot2 will include a legend indicating what type of theta the colours are applied to.

Answer 2

As suggested by #Marius, the most efficient way to plot your data is to convert them into a long format.
Using tidyverse, you can have the use of pivot_longer function (from tidyr package) and write the following code:
library(tidyverse)
SWRC_SL %>% pivot_longer(.,-pressure_head, names_to = "variable", values_to = "value") %>%
ggplot(aes(x = pressure_head, y = value, color = variable))+
geom_line()+
scale_x_log10()
EDIT: Illustrating example
Using this dummy dataset:
pressure UNSODA_theta Vrugt_theta Cassel_theta
1 0 -1.4672500 1.4119747 -2.0553118
2 1 0.5210227 0.6189239 1.4817574
3 2 -0.1587546 1.4094018 2.2796175
4 3 1.4645873 2.6888733 -0.4631109
5 4 -0.7660820 2.5865884 -1.8799346
6 5 -0.4302118 0.6690922 0.9633620
First, you pivot your data into a long format:
df %>% pivot_longer(.,-pressure, names_to = "variable", values_to = "value")
# A tibble: 45 x 3
pressure variable value
<int> <chr> <dbl>
1 0 UNSODA_theta -1.47
2 0 Vrugt_theta 1.41
3 0 Cassel_theta -2.06
4 1 UNSODA_theta 0.521
5 1 Vrugt_theta 0.619
6 1 Cassel_theta 1.48
7 2 UNSODA_theta -0.159
8 2 Vrugt_theta 1.41
9 2 Cassel_theta 2.28
10 3 UNSODA_theta 1.46
# … with 35 more rows
Now, your data are suitable for the plotting with ggplot2, you can directly add ggplot command to the previous command by adding a "pipe" (%>%) between them:
library(tidyverse)
df %>% pivot_longer(.,-pressure, names_to = "variable", values_to = "value") %>%
ggplot(aes(x = pressure, y = value, color = variable))+
geom_line()+
scale_x_log10()
And you get the following plot with legend included:
Data example
structure(list(pressure = 0:14, UNSODA_theta = c(-1.46725002909224,
0.521022742648139, -0.158754604716016, 1.4645873119698, -0.766081999604665,
-0.430211753928547, -0.926109497377437, -0.17710396143654, 0.402011779486338,
-0.731748173119606, 0.830373167981674, -1.20808278630446, -1.04798441280774,
1.44115770684428, -1.01584746530465), Vrugt_theta = c(1.41197471231751,
0.61892394889108, 1.40940183965093, 2.68887328620405, 2.58658843344197,
0.669092199317234, -1.28523553529247, 3.49766158983416, 1.66706616676549,
1.5413273359637, 0.986600476854091, 1.51010842295293, 0.835624168230333,
1.42069464325451, 0.599753256022356), Cassel_theta = c(-2.05531181632119,
1.48175740118232, 2.27961753824932, -0.46311085383842, -1.87993463341154,
0.963361958516736, -0.0670637053409687, -2.59982761023726, 0.00319778952040447,
-0.945450500892219, -0.511452869790608, -1.73485854395378, 2.7047128618762,
-0.496698054586832, -2.40827011837962)), class = "data.frame", row.names = c(NA,
-15L))