Visualize ordered continuous data with a line chart

You want to visualize the relationship between two continuous variables with a line chart. Usually, one of your variables is monotonically increasing, like a date or a time.

Step 1 - Pass your data to ggplot2::ggplot(). ggplot() creates a blank canvas for your plot.

Step 2 - Map variables to the the \(x\) and \(y\) axes with mapping = aes(x = , y = ). If one of your variables is a date or time, place it on the \(x\) axis.

Step 3 - Add a layer of lines with ggplot2::geom_line(). geom_line() will draw a single line connecting all data points from left to right along the x-axis.

Step 4 (Optional) - Use mapping = aes() to define groups. If you map a grouping variable to group, color, linetype, size or alpha, geom_line() will draw a separate line for each value of the grouping variable.

ggplot(data, mapping = aes(x = col_A, y = col_B)) +
  geom_line() 

Add data point markers

Add a second layer of points to identify the data points along the line, e.g.

ggplot(data, mapping = aes(x = col_A, y = col_B)) +
  geom_line() +
  geom_point()

Example

uber_fenway contains hourly summaries of Uber rideshare services for one day in the Fenway neighborhood of Boston, Massachusetts.

uber_fenway

# A tibble: 72 × 3
    hour price_mean service 
   <int>      <dbl> <chr>   
 1     0       9.95 UberPool
 2     0      10.6  UberX   
 3     0      18.1  UberXL  
 4     1       9.08 UberPool
 5     1      10    UberX   
 6     1      17.8  UberXL  
 7     2       8.82 UberPool
 8     2      10.4  UberX   
 9     2      16.8  UberXL  
10     3       9.56 UberPool
# ℹ 62 more rows

We want to know how the price of Uber rides changes throughout the day in Fenway.

To explore this, we visualize the relationship between price_mean and hour as a line chart. We use color to create a separate line for each type of Uber service.

library(ggplot2)
ggplot(
  uber_fenway, 
  mapping = aes(x = hour, y = price_mean, color = service)
 ) +
  geom_line()

Respect groups

If your line chart suffers from a whipsaw effect, as below, it is likely that you are trying to plot multiple groups with a single line. Map a grouping variable to color or group to restore order.

ggplot(uber_fenway, mapping = aes(x = hour, y = price_mean)) +
  geom_line()

Line charts in SAS

ggplot() with geom_line() and geom_point() is the equivalent of SAS’s SGPLOT procedure with the SERIES statement:

In SAS:

PROC SGPLOT DATA = data_plot; 
  VBAR X = col_A Y = col_B / MARKERS;
RUN;

In R:

ggplot(data_plot, mapping = aes(x = col_A, y = col_B)) +
  geom_line() +
  geom_point()