Chart prompts: choose marks and scales by data type

Last updated: 4 Oct 202611 min read

tutorial

A prompt for chart creation should map each field to a visual channel with a reason. Ordered time points can use positions along a time axis; independent categories can use bars or dots. The chosen mark should preserve the comparison the reader needs. State the scale domain and whether zero is required for a magnitude comparison. A truncated bar axis can exaggerate a small difference; a nonzero line-axis baseline may be acceptable when clearly labeled and used to inspect small variation. Avoid dual axes unless their units and relationship are explicit. Color should carry a named category or status, not decorate the page.

Operational case

Container C-47's hourly readings are ordered observations. The chart uses points connected only across adjacent observed hours. It places time on the horizontal axis, degrees Celsius on the vertical axis, and a labeled 5.0-degree threshold. The missing third hour breaks the connection; no line segment crosses that gap. A separate category comparison of depot counts would use bars with a zero baseline. The prompt does not request a pie chart for hourly values or a second axis for an unrelated humidity series.

Mark: point plus line for adjacent observed readings.
X: scheduled hour in local time; Y: temperature in °C.
Threshold: labeled 5.0 °C reference line.
Missing hour: visible gap, not an interpolated mark.
No dual axis and no unlabelled scale truncation.

Performance and operating cost

Rendering N marks is generally O(N), while adding too many series increases visual search and legend review. A long sequence may need downsampling, but the method must preserve peaks relevant to the threshold. Encoding review is a low-cost gate: inspect whether the field type, unit, baseline, and mark agree before drawing. If the model invents a chart type first and maps data afterward, a misleading scale can survive even when every underlying value is correct.

Common Mistakes

Do not join points across a missing interval.
Do not use a nonzero bar baseline for an ordinary magnitude comparison.
Do not encode an unlabelled second unit on the same axis.

Connected lessons

prompt engineering

chart creation