an optional variable name, if entered the plot will be created between a1 and a2
type
either 'r' for square-root differences or 's' for squared differences
query.a
an optional variable name, if entered, the value of the variable will be displayed on the graphics device for points
identified by the user.
...
additional arguments for plot
Value
NULL
Note
When query.a is entered, the user will be prompted to identify points on the display device. Because each point in the
plot represents a pair of locations, the user must identify each point twice, once for the "from" point and once for the "to"
point. Querying is ended by pressing the middle mouse button on the mouse while the cursor is in the display window.