data.col:equally-spaced d col {threshold 1e-6}
| d | data object |
| col | the column to examine |
| threshold | if absolute differences between all steps are less than the specified threshold times the first step size, steps are regarded as equal. default=1e-6 |
| returns 1 if the column is equally-spaced, or 0 otherwise |
| if data length is less than 3, returns 1 |
if {[d.col equally-spaced x]} {
set dx [expr [d get 1 x] - [d get 0 x]]
}