Read a scatterplot

Reading a scatterplot

A scatterplot places the explanatory variable on the horizontal axis and the response variable on the vertical axis. Each case contributes a point, but cases with identical pairs overlap. Read a coordinate pair as horizontal value followed by vertical value with any stated scale applied

1. Use the horizontal axis The explanatory variable is placed on the horizontal axis of a scatterplot \(\displaystyle x=\text{EV}\)
2. Use the vertical axis The response variable is placed on the vertical axis \(\displaystyle y=\text{RV}\)
3. Count carefully Distinct markers equal cases only when no observations coincide. Otherwise check multiplicities or the source table
4. Apply the axis scale Convert a printed coordinate when the axis is labelled in hundreds, thousands or another scale \(\displaystyle 4\text{ thousand}=4\times1000=4000\)

Worked example

A scatterplot relates elevation, measured in hundreds of metres, to air pressure in hectopascals. The point with the largest elevation is plotted at (8, 940). Find its actual coordinate pair, with elevation first

1. \(\displaystyle x=\text{elevation},\quad y=\text{air pressure}\)Elevation is the explanatory variable, so it is read on the horizontal axis
2. \(\displaystyle 8\times100=800\text{ m}\)The horizontal value 8 represents 800 metres because the axis is in hundreds of metres
3. \(\displaystyle \left(\text{elevation},\text{air pressure}\right)=\left(800,940\right)\)Read the vertical value as 940 hectopascals and write the horizontal coordinate first

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