Data Visualisation and Reporting
Match the chart to the question
Chart choice follows the question and the data types. Bar/column: compare values across categories. Line: change over time or another ordered continuum. Pie/doughnut: parts of a single whole - only with few slices (roughly five or fewer) and non-negative values. Scatter: relationship between two numeric variables, revealing correlation, clusters and outliers. Histogram: distribution of one continuous numeric variable (bins, no gaps between bars). Box plot: spread, median and outliers across groups. Stacked bar: composition across categories, though inner segments are hard to compare.
Honest design
Bar charts should start the value axis at zero, because bar length encodes the value; a truncated axis exaggerates differences. Line charts may use a non-zero axis when the change is genuinely small, but the axis must be labelled clearly. Avoid 3D effects, exploded pies, rainbow palettes, redundant gridlines and decoration: maximise data-ink. Always include a title stating the message, axis labels with units, a legend where needed, the time period and the data source. Use colour meaningfully (one highlight colour against grey), keep it accessible for colour-blind readers (do not rely on red/green alone) and label values directly where space allows.
Dashboards and reports
A dashboard is a single-screen, interactive summary for monitoring: consistent scales, filters or slicers, and a small number of key metrics. A written report adds narrative: audience and purpose, executive summary, method, findings, limitations and recommendations. Both must protect privacy - aggregate small groups so individuals cannot be re-identified.
Traps
Pie charts for time series or with many slices; line charts joining unordered categories; confusing a histogram with a bar chart; dual y-axes implying a relationship; truncated axes on bars; missing units; titles that overstate what the data shows.
Rule of thumb
Write the sentence you want the reader to take away first, then pick the chart that makes that sentence easiest to see - and check the axis before trusting the picture.