Showing how something moved over time.

The most-used chart form and the one with the most implied claims. A line says the values between the points would have been on the line.

Drawn as a line chart, area chart, time series, depending on the data.

The work did not stop. The money did.

Rendered from the project Business account — 12 months, through the same compiler the product uses. Not a picture of one.

What it is for

A trend chart answers whether something is moving and in which direction. It works because position along a common axis is the most accurately read visual encoding there is, and because a line makes the direction of travel legible at a glance rather than by comparison.

The claim a line makes is continuity. Joining two points asserts that the quantity existed between them and passed through the values on the segment. That is true of a daily balance and false of a monthly survey score, and using the same mark for both is the most common way a trend chart overstates what is known.

Reach for it when

The x axis is time, and the intervals are regular
You care about direction and shape rather than about comparing individual values
There are enough points that the shape means something, which is usually more than five
The quantity genuinely exists continuously between the points

Use something else when

The categories are not ordered, since a line between unordered categories asserts a sequence that does not exist
There are two or three points, where a line dramatises a difference better shown as values
The final period is incomplete, unless it is visibly marked as partial

How this form misleads

A partial final period drawn like a complete one

Month-to-date against full months is the single most common way a chart shows a collapse that is not happening. The line falls off a cliff on the last point and the cliff is the calendar.

Either exclude the incomplete period or draw it distinctly and label it. A dashed final segment with the day count is enough.

Truncating the y axis to make a change look bigger

For a line chart a zero baseline is not always required, since the subject is change rather than magnitude. But a heavily truncated axis converts a 2% move into a visual crash, and the reader's impression is set by the shape rather than by the labels.

Choose the range from the data's own variation, show the axis labels clearly, and if the point is that a change is small, let it look small.

Too many series on one chart

Above about five lines, the chart becomes a lookup table with extra steps, and the reader traces individual lines rather than seeing a shape.

Highlight the one or two series the finding is about and render the rest as context in a single neutral tone, or split into small multiples.

What this will not draw, whatever you ask for

The agent picks an intent from a closed list and a single compiler renders it. These forms are rejected at the schema layer rather than discouraged in a style guide, which is the difference between a rule and a preference.

3D anything: Perspective makes two equal values look different. There is no case where the third dimension carries data.
Gauges and speedometers: An enormous amount of ink for one number, and the dial implies a range that is usually invented.
Radar charts: Area scales with the square of the values and the shape changes entirely if you reorder the axes.
Word clouds: Size encodes frequency, which is the least interesting thing about a word, and nothing is comparable.
Pie charts above five slices: Angles are hard to compare, and beyond five slices the labels leave the chart anyway.
Per-series arbitrary colour: Colour means something specific in this product. A series that picks its own hue breaks the meaning everywhere else.

Make one from your own data

You do not choose the form. The analysis works out which question your data can answer and picks the chart that answers it, then tells you why.

No account needed to start. You only pay when you like what you see.