Baseline

A baseline is the reference value — normally zero — from which a chart measures the height of bars, columns and area fills.

When you draw a bar, its length is not arbitrary: it represents the distance between the baseline and the data value. Choose zero as the baseline and a bar's length is directly proportional to its value, so a bar twice as long means twice as much. The baseline is therefore the silent assumption behind every length comparison a reader makes.

How the baseline works in a chart

On most bar and column charts the baseline sits at the bottom (or left) edge where the value axis reads zero. It is where every bar starts. In a few chart styles the baseline is moved deliberately — for example a chart showing deviations might use a baseline at the average so bars point up for above and down for below. But moving it without telling the reader is one of the most common ways to distort a chart.

A concrete example

Suppose two columns represent 102 and 108. With a baseline at 0, the second column is only about 6% taller — they look almost equal, which is honest. Now move the baseline up to 100. The first bar becomes 2 units tall and the second 8 units tall, so the second looks four times bigger even though the real values differ by under 6%. Same data, wildly different impression — all because the baseline shifted.

Related terms

The baseline lives on the value axis and depends on its scale; the lowest tick mark usually sits at the baseline value. This matters most for length-based charts — see the bar chart guide for why zero baselines are the default.