Line charts
What are they?
Line charts show trends and change over time for one or more categories. The horizontal axis often shows time and the vertical axis shows what is being measured. Two other line chart types are area charts and slope charts.
Consider other chart types before using
Consider other chart types before using charts with this label. They might be appropriate, but less familiar charts are often misinterpreted—book office hours with the data viz team if you’re unsure.
Line Chart Guidelines
Gridlines
Provide enough gridlines to help readers interpret the data points. Use logical increments. Don’t overcrowd the chart with more gridlines than necessary and avoid gridlines in unusual increments.
No spaghetti charts!
In most cases, only include up to 4 lines in a single chart. Try to never use more than 6 lines. It is difficult to see each line or make comparisons when there are many lines in the chart. Consider small multiples instead.
Line thickness
Lines should be thick enough to stand out from the gridlines, but thin enough to show detail in twists and turns. Lines that are too thick can hide the small changes between the data points.
Use a different line treatment or background to show future data
If you are showing future data, indicate that change to the reader. One way is to use a dashed line in the same color as the past data. Make sure to add a legend entry for that dotted line. A solid line in a lighter shade could work as well. An alternative is to shade and label the background.
Use color to highlight the most important aspect of the story
If one of the lines in a chart is more important than the others, you can use color to make that single line stand out from the others. Gray can be used to deemphasize lines that are shown for context but are not part of the main story.
