Extracting positions (x and y coordinates) of a drawn grid grob

[Anirban166]

Hi @pmur002
I was wondering whether we could acquire the (x, y) positions of a grid grob after it has been drawn (i.e. the x and y coordinates in pairs resembling the points which altogether construct that particular grob) - if that's possible, what would be the approach?

What I'm trying to achieve is to obtain

• the y-position/coordinate of the topmost part of a drawn polygon grob (any one point along the topmost edge would suffice, since while the x-coordinate will vary, the y-coordinate will be constant, being the maximum value for the points on the top side of the polygon - I would like this y_max value)
• and the topmost line of the plot panel (creating the top-level boundary for the plot).

Link for reference: tdhock/directlabels#41 (comment)

[pmur002]

Are you looking for grid::grobX() and grid::grobY() ?

[Anirban166]

I tried using them but they aren't returning anything useful: (just returns the name of 'rotationgrobaxis', where rotation is the angle equivalent of the direction-based theta value provided, and axis is either x or y depending upon use of grobX or grobY)

> df <- data.frame(i=1:10, label="more
+ than
+ one
+ line
+ really
+ a
+ lot")
> library(ggplot2)
> gg <- ggplot(df, aes(i, i, color=label)) + geom_line()
> library(directlabels)
> direct.label(gg, "right.polygons")
> library(grid)
> grid.force()
> grid.ls(fullNames = TRUE)
forcedgrob[layout]
  forcedgrob[background.1-9-12-1]
  forcedgrob[panel.7-5-7-5]
    gTree[grill.gTree.14]
      rect[panel.background..rect.5]
      polyline[panel.grid.minor.y..polyline.7]
      polyline[panel.grid.minor.x..polyline.9]
      polyline[panel.grid.major.y..polyline.11]
      polyline[panel.grid.major.x..polyline.13]
    zeroGrob[NULL]
    polyline[GRID.polyline.1]
    dlgrob[GRID.dlgrob.1.right.polygons]
    zeroGrob[NULL]
    zeroGrob[panel.border..zeroGrob.2]
  forcedgrob[spacer.8-6-8-6]
  forcedgrob[spacer.8-4-8-4]
  forcedgrob[spacer.6-6-6-6]
  forcedgrob[spacer.6-4-6-4]
  forcedgrob[axis-t.6-5-6-5]
  forcedgrob[axis-l.7-4-7-4]
    zeroGrob[NULL]
    forcedgrob[axis]
      forcedgrob[axis.1-1-1-1]
        text[GRID.text.22]
      forcedgrob[axis.1-2-1-2]
  forcedgrob[axis-r.7-6-7-6]
  forcedgrob[axis-b.8-5-8-5]
    zeroGrob[NULL]
    forcedgrob[axis]
      forcedgrob[axis.1-1-1-1]
      forcedgrob[axis.2-1-2-1]
        text[GRID.text.17]
  forcedgrob[xlab-t.5-5-5-5]
  forcedgrob[xlab-b.9-5-9-5]
    text[GRID.text.26]
  forcedgrob[ylab-l.7-3-7-3]
    text[GRID.text.30]
  forcedgrob[ylab-r.7-7-7-7]
  forcedgrob[subtitle.4-5-4-5]
  forcedgrob[title.3-5-3-5]
  forcedgrob[caption.10-5-10-5]
  forcedgrob[tag.2-2-2-2]
> y1 <- grid::grobY("GRID.dlgrob.1.right.polygons","north")
> y1
[1] 90groby 
> class(y1)
[1] "unit"    "unit_v2"
> y2 <- grid::grobY("panel.grid.major.x..polyline.13","north")
> y2
[1] 90groby 

Also, as you can see from the above they're returning the exact same thing for two different grobs. Am I doing something wrong?

[pmur002]

It depends on your definition of "useful" :) y1 and y2 are "unit" objects, so they can be used as locations or dimensions for other 'grid' drawing, e.g., ...

grid.points(unit(.5, "npc"), y1)

... or they can be converted to other coordinate systems, e.g., ...

convertY(y1, "in")

... with the warning that the value of the y1 unit will make most sense in the context of the grob it is referring to, i.e., "GRID.dlgrob.1.right.polygons" (and is likely to be completely nonsensical in other contexts).

[Anirban166]

Ah gotcha. I was too quick to assume that they were not useful, but it seems like I can make some use out of them now :)
will update you when I do - might be a bit late though, sorry! (closing this for now)