Data Analyst & Analytics Interview · Data Visualization & BI · Lesson 1 of 2
Chart choice & BI concepts
The right chart makes the insight obvious; the wrong one hides it. Match the chart to the question, not to what looks impressive.
- Trend over time → line chart.
- Compare categories → bar chart (horizontal for long labels). Avoid pie charts beyond 2-3 slices.
- Relationship between two numbers → scatter plot.
- Distribution → histogram or box plot.
- Part-to-whole over time → stacked area (sparingly) or small multiples.
- Single KPI → big-number tile with a comparison (vs target / prior period).
BI tool concepts
- Dimensions vs measures: dimensions are categorical (region, date); measures are aggregatable numbers (revenue). This split is the mental model behind Tableau and Power BI.
- Filters, drill-down, and slicers let users explore without new queries.
- Calculated fields / DAX (Power BI) and LOD expressions (Tableau) define metrics once, reuse everywhere.
- Live vs extract: query the warehouse in real time, or cache an extract for speed. Snowflake/BigQuery are common backends.
Chart crimesTruncated y-axes exaggerate tiny changes; dual axes imply false correlation; 3-D and pie charts distort comparison; rainbow color scales mislead. Start bar/line axes at zero unless you have a strong reason and a label.
Encoding ranksPeople read position and length far more accurately than angle, area, or color. That is the real reason a bar chart beats a pie: bars encode value as length, which the eye compares precisely.
◆ Lock it in
- Match chart to question: line=time, bar=compare, scatter=relationship, histogram=distribution.
- BI rests on dimensions vs measures; define metrics once as calculated fields.
- Avoid chart crimes: zero baselines, no 3-D/pie overload, honest axes.
Feynman drill — say it out loudExplain why a bar chart is easier to read accurately than a pie chart, in terms of how the eye perceives length vs angle.
Step 1 rate your confidence · Step 2 pick your answer
You want to show how monthly revenue has changed over the past two years. Best default chart?
Step 1 — how sure are you?