What makes a good data visualisation, and how do you choose the right chart?

Choose the chart from the relationship you are showing:

• Change over time — line chart. Area chart for cumulative totals.
• Comparison across categories — bar chart. Horizontal bars when labels are long, and sorted by value unless there is a natural order.
• Part of a whole — stacked bar, or a pie chart only with very few slices. Beyond three or four segments a pie is unreadable and a bar chart is better.
• Relationship between two variables — scatter plot.
• Distribution — histogram or box plot. This is important and underused: an average alone hides the shape entirely.
• Two dimensions plus intensity — heatmap.
• A single key number — just show the number, large. A gauge chart adds nothing.

What makes it good:

• Title it with the finding, not the contents. "Mobile conversion fell 40% after the redesign" beats "Conversion by Device".
• Start bar chart axes at zero. Truncating exaggerates differences and is genuinely misleading; line charts showing change may reasonably not.
• Remove everything that is not information — 3D effects, heavy gridlines, decorative colour, and redundant legends.
• Use colour to mean something, and check it works for colour-blind readers and in greyscale.
• Label directly rather than forcing a trip to a legend.