Computes the index of the peak velocity of a trajectory, defined by vectors of x and y coordinates, and assumed to be equidistant in time.

## References

Wirth, R., Foerster, A., Kunde, W., & Pfister, R. (2020). Design choices: Empirical recommendations for designing two-dimensional finger tracking experiments. Behavior Research Methods, 52, 2394 - 2416. doi:10.3758/s13428-020-01409-0

## Examples

```
x_vals <- c(0, 1, 2, 3, 6, 10, 12, 14, 15)
y_vals <- c(0, 0, 0, 0, 0, 0, 0, 0, 0)
index_max_velocity(x_vals, y_vals)
#> [1] 5
# velocity maximal between x_vals[5] and x_vals[6]
numbers <- seq(-(3 / 4) * pi, (3 / 4) * pi, by = 0.001)
y_vector <- sin(numbers)
plot(numbers, y_vector)
index_max_velocity(rep(0, length(numbers)), y_vector)
#> [1] 2357
abline(v = numbers[index_max_velocity(rep(0, length(numbers)), y_vector)])
which.max(cos(numbers)) # first derivative of sin, max at 0 degrees
#> [1] 2357
```