Measures the distance between two points which the user has clicked on.
clickdist()A single nonnegative number.
This function allows the user to measure the distance between two spatial locations, interactively, by clicking on the screen display.
When clickdist() is called, the user is expected to
click two points in the current graphics device. The distance
between these points will be returned.
This function uses the R command locator to
input the mouse clicks. It only works on screen devices such as
‘X11’, ‘windows’ and ‘quartz’.