Line graph maker

Joining two points with a line says that the thing being measured passed through the values in between. That is true of a temperature through a day and false of a bar chart of countries, which is the whole difference between the two charts. Use a line when the x axis is an order you moved along: time, distance, trial number, dose.

Because the line makes that claim, a gap in the data has to stay a gap. A month with no figure leaves a break in the line rather than a straight segment jumping over it — a segment would say the value moved smoothly from one side to the other, which is exactly what you do not know.

Columns after the first are drawn as separate lines against the same axis, each with its own color, a legend, and — when there are four or fewer — its name written at the end of the line, so the reader never has to look between a key and a curve. Eight lines is the limit; past that the colors would repeat and two lines would look like one, so the page draws the first eight and says so.

All the lines share one value axis. Two measurements on wildly different scales — revenue in millions and headcount in tens — do not belong on one chart: the small one flattens into the axis. Draw them as two charts, or convert both to an index where each starts at 100.

Unlike a bar chart, a line graph does not have to start at zero, and often should not: a line is read by its slope rather than by its height, and forcing zero onto a chart of body temperature or exam averages squashes the interesting part into a flat band. The option is there for the cases where the distance from zero is the point — and where it is, leaving it off overstates every wiggle.