Check a chart
Type in the axis range a chart used and the two values it plotted. The tool draws the chart as it was shown, then draws it again with an honest, zero based axis, so you can compare them side by side.
Chart as drawn
Honest version, axis starts at zero
Gallery of real tricks
Finance headlines lean on this trick often. Each card below is a realistic example. Click one to load it into the checker above and see the full comparison.
How the math works
Actual difference compares the two values directly: (second minus first) divided by first. This is what the numbers really say.
Perceived difference compares the two bar heights as drawn on the chart's own axis. A bar's height is (value minus axis minimum) divided by (axis maximum minus axis minimum). When the axis does not start at zero, this height ratio no longer matches the value ratio, and the gap looks bigger than it is.
When the axis starts at zero, bar height and value are proportional, so perceived and actual match exactly. That is what makes a zero based axis honest by construction.
This is a documented effect, not a guess. Research on y-axis truncation found viewers keep overestimating the difference even after being taught how the trick works, with 83.5% of participants across five studies still showing a truncation effect (ScienceDirect). Knowing the rule does not fully protect you, which is why checking the actual numbers matters.