Interval-scaled data is perhaps the most natural data type (at least for computer scientists and mathematicians). This data type implies continuous data values which do indeed follow a linear scale.
Data of this format will have the following invariant property:
"X is five inches taller than y" has meaning regardless of the values of x and y.
Data of this type is easily mapped to many graphical parameters, and
thus is most natural in visualization applications.
To the right, we see a straightforward graph of interval data.