Quantities and zeros
Most tables contain values that are quantities — values in which zero has special meaning. Ideally, when a bar chart is used to represent the data, the bars should all extend up from zero, so that the amount of ink on the paper is proportional to the quantity.
Bar lengths should be proportional to the values.
Some data sets contain values that are very close together and all bars would visually seem to be the same if a bar chart was drawn properly. There is then no alternative but to violate this principle in order to graphically represent the data. A bar chart can be drawn to a scale that starts at a value higher than zero, but it is important to clearly indicate that the scale no longer starts at zero with zig-zags at the bottom of the axis and/or bars.
Life expectancy in Western Europe
The bar chart below shows life expectancies for males in some of the larger countries in Western Europe in 2003.
Since all values are between 74 and 78, the bars all have similar heights and the bar chart does not effectively show differences between the countries. Click Expand scale to change the vertical scale so that it starts at 70 instead of zero.
The resulting scale distinguishes the countries better, but note the use of zig-zags at the bottom of the bars to highlight the fact that they no longer start at zero.
Negative values
Some data sets describe quantities that can be negative. In particular, many demographic and economic statistics can increase or decrease from year to year, so their changes can take either sign.
Such quantities can be represented on a bar chart with bars extending on both sides of zero. It is best to use a different colour or shading for the bars representing the negative values so that the negative values stand out.
Projected population changes in New Zealand
The next bar chart shows projected regional population changes in the period 2001-2026. The population is projected to decrease in some regions, so their bars ard drawn to the left of zero.