Data Analysis

In-Class Exercise 3

Yu-You Liou

Shih Chien University

2026-08-03

Question 1

Using the economics dataset, make two plots of the unemployment rate over time (x = date, y = unemploy / pop): one with geom_line() and one with geom_point(). Then make a bar chart of the number of cars in each class of the mpg dataset using geom_bar().

Write one sentence explaining why a line is the right geom for the unemployment series but a bar is the right geom for the class counts.

Question 2

Using the economics dataset, plot unemploy / pop against uempmed (median duration of unemployment) with geom_path(). Compare it with the same plot drawn with geom_line().

Write one sentence explaining the difference: which geom connects observations in the order they appear in the data (time order), and why that matters for showing how the economy evolved.

Question 3

Using the mpg dataset, plot displ vs hwy with one smooth line per vehicle class. First try mapping colour = class inside aes(), then try a version where the points are a single colour but you add geom_smooth(aes(group = class), se = FALSE). Combine points and smooths as multiple geoms in one plot.

Write one sentence explaining what the group aesthetic does when you do not want a separate colour (or legend) for each group.

Submission

Complete your answers in the official template from the Problem sets page (author = your student ID only; keep the UUID chunk). Knit to PDF, name it ex3-[StudentID].pdf, and upload it to the course pCloud folder before midnight of the following week.