19.2 Scatter plot

# Example of a scatter plot: add the geom_point() layer
ggplot(data=dataframe, mapping=aes(x=column1, y=column2)) + geom_point()

• Example of a simple scatter plot:

# Create a data frame
df1 <- data.frame(sample1=rnorm(200), sample2=rnorm(200))

# Plot !
ggplot(data= df1 , mapping=aes(x=sample1, y=sample2)) + 
    geom_point()

• Add layers to that object to customize the plot:
  • ggtitle to add a title
  • geom_vline to add a vertical line
  • etc.

ggplot(data= df1 , mapping=aes(x=sample1, y=sample2)) + 
  geom_point() +
    ggtitle(label="my first ggplot") +
    geom_vline(xintercept=0)

Bookmark that ggplot2 reference and that good cheatsheet for some of the ggplot2 options.

• You can save the plot in an object at any time and add layers to that object:

# Save in an object
p <- ggplot(data= df1 , mapping=aes(x=sample1, y=sample2)) +
    geom_point()
# Add layers to that object
p + ggtitle(label="my first ggplot")

• What is inside the aes (aesthetics)function ?
  • Anything that varies according to your data !
    • Columns with values to be plotted.
    • Columns with which you want to, for example, color the points.

Color all points in red (not depending on the data):

ggplot(data=df1 , mapping=aes(x=sample1, y=sample2)) +
    geom_point(color="red") 

Color the points according to another column in the data frame:

# Build a data frame from df1: add a column with "yes" and "no"
df2 <- data.frame(df1, grouping=rep(c("yes", "no"), c(80, 120)))

# Plot and add the color parameter in the aes():
pscat <- ggplot(data=df2, mapping=aes(x=sample1, y=sample2, color=grouping)) + 
  geom_point()