Graphs and Charts
A table of 200 rents tells you almost nothing at a glance. The right chart shows the pattern in two seconds: a long tail, an outlier, a trend. The wrong chart hides it or, worse, invents a pattern that is not there. Choosing well is a small, repeatable decision.
The rule
Pick the chart from the question: one variable, two variables, groups, or time.
Each common chart answers one kind of question:
- Histogram: how is one numeric variable spread? Bars count how many rents fall in each band (10–15, 15–20, and so on). It shows the shape: symmetric, skewed, two humps.
- Box plot: a compact view of one numeric variable. The box runs from the first to the third quartile, a line marks the median, and dots beyond the whiskers flag outliers like the penthouse at 80. Several box plots side by side compare groups.
- Bar chart: one number per category, such as the average rent for 1-, 2- and 3-room flats.
- Scatter plot: two numeric variables, one dot per item. Size against rent for our five flats shows whether bigger flats cost more.
- Line chart: a number over time, such as average rent each month.
import matplotlib.pyplot as plt
sizes = [40, 60, 80, 50, 70]
rents = [14, 22, 26, 16, 22]
plt.scatter(sizes, rents)
plt.xlabel("Size (m²)")
plt.ylabel("Rent (₹ thousand per month)")
plt.show()Honesty rules that apply to every chart:
- Label both axes, with units.
- Start bar charts at zero. A bar's length is the message; cutting the axis at 18 makes 20 look twice as big as 19.
- Avoid pie charts with many slices; people compare lengths far better than angles.
The shortcut
Count the variables and name their types (number, category, time), then read off the chart.
| What you have | Chart |
|---|---|
| One number | Histogram or box plot |
| One category | Bar chart of counts |
| Number against number | Scatter plot |
| Number across categories | Box plots or bar chart |
| Number over time | Line chart |
Are most rents cheap with a few very high?
One number, its spread: a histogram, or a box plot to flag the outliers.
Drills
Answer fast — trust the rule.
The agency asks, "Does rent rise with size?" Which chart answers it?
Which chart shows how the average rent changed month by month over three years?
You want the median rent of 200 flats and the unusually expensive ones flagged, in one small chart. Which do you use?
A bar chart of average rent for 1-, 2- and 3-room flats has a y-axis running from 18 to 26. What is wrong?
Remember
- Pick the chart from the question: one variable, two variables, groups, or time.
- Shortcut: count the variables, name their types, read off the table.
- Trap: bar charts that do not start at zero, and unlabelled axes.