Heartworm from Mississippi to North Dakota, flattened by a 0–100% axis
Showing the misleading chart
A dog road-trip newsletter we drew puts 2025 heartworm test positivity for all 50 states and DC on a percentage axis that runs the full 0–100%, and every state lands in the same thin band along the floor. The rates run from 0.21% in North Dakota to 6.20% in Mississippi, about 1 positive test in 475 against 1 in 16, and five states returned 41% of the country’s positives from 12% of its tests. Range the axis to the data and the flat line opens into a 29-fold spread.
01The claim
Heartworm is a fewer-than-1-in-100 problem everywhere in the country, so where you take your dog makes no difference and a road trip needs no extra planning for it.
02The trick
Every column is CAPC’s own 2025 figure, the baseline is zero, the ticks are even and no state is missing. The trouble is the top of the axis: it runs to 100% because the quantity is a percentage, not because any value comes near it. The highest state is 6.20%, so the whole field is squeezed under the 7% mark, and the gap between the best state and the worst, 6 points, takes up six percent of the chart’s height. At that size the eye can’t tell a 6.20% column from a 0.21% one; it sees 51 near-identical slivers and reads the states as alike. They aren’t: in Mississippi about 1 test in 16 came back positive (8,513 of 137,254), in North Dakota about 1 in 475 (60 of 28,512), a 29-fold difference. Nothing in the drawing is wrong; the conclusion comes from the 94% of the frame that holds no data. (The Long Leash and its advice are our invention, drawn to show the mechanism; every figure is CAPC’s.)
03The fix
The redraw keeps the zero but ranges the axis to 0–7%, just above the highest value, and draws each state as a dot on a thin stem, sorted, so all 51 labels fit. The flat band spreads out: nine states at 1.5% or more, all in the South, and 32 of the 51 at 0.50% or below. Mississippi, Louisiana, Arkansas, Alabama and Texas ran 12% of the year’s tests and returned 41% of its positives. Printing “1 in 16” and “1 in 475” beside the percentages helps too, because small rates are easier to compare as odds. Two cautions keep the redraw honest. These are positive tests among dogs brought to a clinic, not the share of all dogs infected, and CAPC doesn’t record why each test was ordered. And low is not zero: every state returned positives, and the American Heartworm Society recommends year-round prevention and an annual test for every dog wherever it lives. What the geography changes is how common infection is among the dogs tested there, and so how much a missed dose could matter, which is exactly what the 0–100% frame made invisible.