Bullet vs Gauge

The gauge chart is an oft-maligned chart type. It has a poor data-to-ink ratio and it's difficult to interpret accurately. But I think it has some merit. Firstly, it is a very familiar chart type that most people can interpret it consistently. After all, it has been used on car dashboards since the Model-T.

Gauges also have value when on a mobile dashboard displaying a single metric. The gauge uses more vertical real estate and it still difficult to interpret accurately. But I have found it is less confusing than a bullet chart in this instance. Bullet charts are hard to read because the origin point is confusing without multiple reference bars to compare. Furthermore, bullet charts are generally more confusing than gauges because they are new to most people. But, most people have seen a gauge and can consistently interpret the bands and reference points. Also a bullet charts require more horizontal real estate which is more valuable on a mobile device than a desktop since the aspect ratios are flipped.

Here is a side-by-side comparison of both chart types in a mobile dashboard in Tableau. See my review of the strengths and weaknesses below. Please let me know your thoughts below.

In my opinion both options have their strengths and weaknesses.

Bullet Chart Dashboard
   + Bullet chart is more accurate because you are comparing distance not angle
   + Requires less vertical real estate because the gauge chart is taller
    Bullet charts are unfamiliar to most people and the bands can be confusing
    A single metric looks odd and it's not obvious you read the bar left-to-right
    Requires more horizontal real estate because bullet is longer

Gauge Chart Dashboard
   + Gauge chart is more common meaning consistent interpretation and fewer questions
   + When only displaying one metric the visual weight of the gauge focuses user's attention on that metric
    Gauge chart is less accurate because you are comparing angle not distance
    Gauges axes are not labeled so inaccurate interpretation is more likely
    Requires more vertical real estate because bullet chart is shorter

This example is inspired by real life dashboard my team and I built at Deloitte. The audience were executives and each dashboard only showed one metric relative to target with metric performance over time. We used a bullet chart and the audience was confused and frustrated. I think the gauge approach is more familiar albeit not best practices.

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