Bar charts encode categorical values by length. By comparing bar lengths, we can visually compare the category sizes.
When a bar is truncated due to a missing zero-baseline or an interrupted y-axis, the relative size difference between the bars changes. Now, the bars no longer visually encodes the actual category value. (Read more from a previous blog)
Misleading bar chart
The above is a DataViz classic. FOX NEWS reported an (to them: alarming) increase in Obamacare enrollments over a few days with a bar chart. They apparently feared imminent bankruptcy of the USA and therfore save the nation by overemphasizing the increase with truncated bar. Instead of the moderate ~20% rise (6 to 7 milllion), their bar showed a 300% increase in length!
Non-misleading representation
I quickly re-designed the chart in Excel. The increase is still clearly visible, even with a zero-baseline.
Same data with zero-baseline. Lower: adjusted axis labeling.
Line or bar chart?
Alternatively, FOX NEWS could have simply used a line chart. Indeed, temporal changes are easier to understand in line charts: our eyes can now follow an upward (increase), or downward (decrease) line. Note: a line chart for 2 time points only is known as a slope chart!
Even better for FOX NEWS: omitting the zero-baseline is possible in line charts as they only communicate relative changes and focus on the trend and not absolute numbers (more here).
Never trust the default!
On a side-note: the default bar chart generated by Excel for this data IS A BAR WITHOUT A ZERO-BASELINE! I am in shock (and obviously don’t use Excel very much)!
So maybe, just maybe FOX NEWS simply used Excel default settings??? Reader, beware: never ever trust a default setting!
helena * jambor 
Nice post, as always.
1. To better convey the notion of time, M27 could/should be written out as in the original.
2. Isn’t the additional rendering of the counts in the “line or barchart” vis, unnecessary, visual clutter? There’s a y-axis already.
3. I find numbers with 5 tailing zeros extremely hard to read. What about “.” separators or “M” as the unit?
4. I don’t agree about skipping the baseline in a slope-plot. Imho the same argument as for a bar plot applies: Without a 0-base, any random fluctuation/trend will always appear as solid effect.
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1. agree fully! I just forgot to adapt…
2. agree, the slope chart is even more minimal, I made a second version and posted it next to the chart!
3. I was toying with this… 1e^10 etc is really hard to read too (esp for non-science people). And, I have bad experience with abbreviations, M can mean too many things! I like the “,” at least..
4. it depends. In order to see trends your eye needs ~ a 45deg angle to visually perceive “change”. If the data is really far from the zero-baseline, then the data could look flat although there was an important increase, e.g. feaver curve!
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