# Chapter 4 Line Plot

Line graph is used to look at the changes in variable over time or look at the relationship between two variable. In both cases, x axis corresponds to the independent variable (time, days etc), y axis corresponds to the dependent variable (temperature, income etc).

## 4.1 How to draw a line plot in base R?

Letâ€™s consider pressure data set being available in R.

``head(pressure) #shows first 6 observations``
``````##   temperature pressure
## 1           0   0.0002
## 2          20   0.0012
## 3          40   0.0060
## 4          60   0.0300
## 5          80   0.0900
## 6         100   0.2700``````
``````plot(pressure\$temperature, pressure\$pressure, type="l",xlab="Temperature",ylab="Pressure",main="Relationship btw Pressure and Temperature") #type="l"
#xlab and ylab options arrange axes name.
# To add points and/or multiple lines (Figure 2-3, right), first call plot() for the first line,
lines(pressure\$temperature, pressure\$pressure/2, col="red") #add red lines
points(pressure\$temperature, pressure\$pressure/2, col="red") #add points to the lines``````

## 4.2 How can we draw line plot in ggplot2?

``qplot(pressure\$temperature, pressure\$pressure, geom="line")``

If the two vectors are already in the same data frame, you can use the following syntax:

``qplot(temperature, pressure, data=pressure, geom="line") #with label name``

Another way

``ggplot(pressure, aes(x=temperature, y=pressure)) + geom_line() #geom_line converts your plot into line plot``

``ggplot(pressure, aes(x=temperature, y=pressure)) + geom_line() + geom_point() #to add points``

``qplot(temperature, pressure, data=pressure, geom=c("line", "point"),col="red") #to change colors of point and line``

This way is the better one.