Computes the angle (in degrees) between a line,
defined by two points with coordinates (`x0`

, `y0`

)
and (`x1`

, `y1`

), and the specified axis.

## Arguments

- x0
x-value of the first point.

- x1
x-value of the second point.

- y0
y-value of the first point.

- y1
y-value of the second point.

- swap_x_y
Whether to compute the angle relative to the x or y axis. Defaults to

`TRUE`

, indicating that the angle is relative to the y axis.

## Details

If the angle is computed relative to the x axis, counterclockwise changes are counted as positive. If the angle is computed relative to the y axis, clockwise changes are counted as positive.

## 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

```
# Note that not the mathematical definition of angle is used by default:
starting_angle(0, 1, 0, 0)
#> [1] 90
starting_angle(0, 1, 0, 0, swap_x_y = FALSE)
#> [1] 0
# angles are clockwise and relative to the y-axis.
# Note that return values are in the range [-180, 180], not [0, 360]:
starting_angle(0, -1, 0, -1)
#> [1] -135
starting_angle(0, 1, 0, -1, swap_x_y = FALSE)
#> [1] -45
```