Line Chart: Good Title, Bad Scale

Charts and graphs are meant to convey information clearly. To convey meaningful information, they must be accurate.

A clear title, but a skewed scale spells propaganda, not information
A clear title, but a skewed scale spells propaganda, not information

I saw this chart on Meet The Press this morning while folding the laundry (sorry David Gregory). Paul Ryan used this chart to illustrate some talking points on the budget, economy, and general political direction of Washington. David was visibly upset about Mr. Ryan injecting his chart into the conversation. That Mr. Ryan had to hold up a chart, rather than NBC making a slide to cut to while he talked, may illustrate that the producers were uncomfortable with the chart.

Perhaps the reason David didn’t want Rep. Ryan to show his chart is that the chart presents a skewed look at the situation, and to call Rep. Ryan on it would eat up time allotted for discussion of the actual topic at hand.

Mr. Ryan’s chart was effective at getting his point across, but it was also a bit misleading, and therefore, not exactly an honest representation of the facts.

Good chart titles tell the viewer what they are looking at

The clean title on this chart is excellent — it puts the bottom line up front, telling the audience exactly what the chart is saying — Spending is the Problem. Pretty clear. It provides context to the viewer so that they can quickly focus-in on the next level of information — the specifics of why.

Too often people write titles like Percentage of the Budget allotted to spending and taxes or worse, Percentage vs. Time

If you only tell viewers what the chart axes are, then you force them to scrutinize the data just to figure out what the trend is.

Bad charts skew scales to exaggerate trends

This chart plots amount over time, which is a good thing for a line chart to plot. The amount, or the y-axis, is in percentage rather than real numbers. This does two things, it keeps some of the lines flat, as they remain a constant rate (yet varying dollars) over time, and it compares parts of a whole, 100% being the whole. The problem with this chart is that the top of the y-axis is 40% rather than 100%. This artificially inflates the slope of the line, making it look steeper.

A better way to do the chart would have been to stretch the y-axis up to 100% and use the white space above to illustrate the shrinking proportion of the budget that can go towards paying down the the debt.

This blog post is not meant as a political statement — it is strictly a statement on using charts and graphs to convey information.

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