The scales below display the measured values for each species of wood compared to a control value from Oak – a wood species that most people are familiar with.
Each red line represents the measured value for that particular species of wood, while the tan line beneath it shows how that value compares to Oak.
In other words, if the red line is very long, that property (such as weight, hardness, etc.) is high relative to Oak. The tan line indicates the baseline for Oak, providing a familiar point of reference.