[R] Mass 'identify' on 2d-plot

Barry Rowlingson B.Rowlingson at lancaster.ac.uk
Mon Dec 5 10:51:08 CET 2005

Evgeniy Kachalin wrote:

> What is ability in R to graphically (per mouse) define some area and to 
> select all the cases felt in it?
> 'identify' is OK for 5-10 cases, but what if cases=1000?

  You can use 'locator' to let the user click a number of points to 
define a polygon, and then use one of the point-in-polygon functions 
provided by one of the spatial packages to work out whats in your polygon.

  Look at splancs, spatstat, sp - pretty much anything beginning with 
'sp' - on CRAN.

  In splancs you can just do:

  poly = getpoly()

  - which lets the user draw a polygon on screen, then:

  inPoly = inpip(xypts,poly)
  points(xypts[inpip,], pch=19,col="red")

  and that will plot the selected points in solid red dots.

  I don't think there's a way to draw a freehand figure on an R plot, 
you have to go click, click, click, and draw straight lines.


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