Axes, Units, Scales and Legends

Every figure is built on choices, and those choices control what the picture can honestly say. The axis label tells you what is being measured. The unit tells you the size of one step. The scale tells you how far apart those steps sit, and whether the axis starts at zero or somewhere else. The legend tells you which line or bar belongs to which thing. Read all four before you read the shape, because the shape alone can mislead.

A line that fills a whole panel looks dramatic, but that drama often comes from the panel's floor, not from the data. If the vertical axis runs from a high starting value to a slightly higher one, any rise looks like it climbs the full height of the chart, even if the real change is a small fraction of the total. The fix is to check the numbers printed at the top and bottom of the axis before judging the slope.

Scale changes the shape even when the numbers are honest. A linear axis spaces equal amounts equally, so repeated halving looks like a curve bending toward zero. A logarithmic axis spaces equal ratios equally, so repeated halving draws a straight line. Neither axis is lying, they just answer different questions: linear shows absolute change, log shows proportional change. This is why two charts of the same costs can look completely different, and why comparing two separate charts requires checking that their vertical scales match before comparing their shapes directly.