Line plot

Overview

Teaching: 20 min
Exercises: 0 min

Questions

Objectives

Let’s load some time series data

data = read.csv('https://raw.githubusercontent.com/vuminhtue/SMU_Python_Visualization/master/data/us.csv')

Convert string to datetime

data$date = as.POSIXct(data$date, format="%Y-%m-%d")

We have loaded covid 19 cases in US for 2020 and 2021. The first column contains the date while second and third are positive cases and number of deaths from Covid19

Line plot for 1 y axis

gp <- ggplot(data, aes(date, cases/10^6))
gp+geom_line()+
  xlab("Date")+
  ylab("Cases (million)")+
  ggtitle("US Covid 19 cases")+
  theme_bw(base_size = 15)

Line plot side by side

```rgp <- ggplot(data, aes(date, cases/10^6)) p1 <- gp+geom_line()+ xlab(“Date”)+ ylab(“Cases (million)”)+ ggtitle(“US Covid 19 cases”)+ theme_bw(base_size = 15)

gp <- ggplot(data,aes(date)) p1 <- gp + geom_line(aes(y=cases/10^6))+ xlab(“Date”)+ ylab(“Cases (million)”)+ ggtitle(“US Covid 19 cases”)+ theme_bw(base_size = 15)

p2 <- gp + geom_line(aes(y=deaths/10^3))+ xlab(“Date”)+ ylab(“Deaths (thousands)”)+ theme_bw(base_size = 15)

library(patchwork) # Thanks to this library p1+p2

![image](https://user-images.githubusercontent.com/43855029/146064760-2e1e6457-67e1-44c4-a411-171fef95c284.png)


## Line plot with 2 axes:

Line plot with 2 axes in ggplot 2 is a little bit tricky. 
Here, we apply the function **scale_y_continuous**, which scale the secondary y-axis to the same as first y-axis.
The tricky part is to define the coefficient value so that the 2 y-axes can be in the same scale.


```r
# First I convert the cases to million and deaths to thousand unit:
cases_mil= data$cases/10^6
deaths_thous = data$deaths/10^3

# Define the coefficient between 2 y-axes:
coef = max(cases_mil)/max(deaths_thous)

gp <- ggplot(data)
gp +  geom_line(aes(date,cases_mil),color="blue")+
  geom_line(aes(date,deaths_thous*coef),color="red")+
  scale_y_continuous(name="Cases (Millions)",
                     sec.axis = sec_axis(~./coef,
                     name="Deaths (Thousands)"))+
  xlab("Date")+
  ggtitle("Covid 19 cases & deaths")+
  theme(axis.title.y.left=element_text(colour="blue"),
    axis.title.y.right=element_text(colour="red")) 

Key Points

Line plot

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