zach charlop-powers

ggrepel with maps

Tue Mar 1, 2016

ggrepel is a package for R that build on ggplot an uses the new ggproto OO system for extending ggplot to provide a few new geom’s for placing text and labels. Its great. I’ve been working on some maps and was hoping I could take the same principles used in ggrepel to add images to a ggplot. In short, I couldn’t find a way that would let you use the ggproto system (unless there is a way to store grobs within data.frames that I don’t know about) but you can call ggrepel directly to find non-overlapping points for arbitrary boxes, and you can add plots, images etc to a grid grapchic using the annotation_custom function of ggplot.

My current results can be found on Rpubs and as a gist Below is a very quick overview of the code.

Flagmap

ggrepelled flagmap

The core idea is to use ggrepel::repel_boxes() to get new coordinates and then plot those coords.


findboxes <- function(df, xcol, ycol, pad_point_x, pad_point_y, xlim, ylim,
                      force = 1e-6, maxiter = 20000) {

    ......

    moved <- ggrepel:::repel_boxes(data_points=as.matrix(posdf), 
                                   pad_point_x=0.1, 
                                   pad_point_y=0.1, 
                                   boxes = boxmatrix,
                                   xlim=xlim,
                                   ylim=ylim,
                                   force=force,
                                   maxiter=maxiter)

    finaldf <- cbind(posdf, moved)
    names(finaldf) <- c("x1","y1","x2","y2")
    return(finaldf)
}

# this can them be used to add a plot
# where grob is an ImageGrob of a PNG file
gg <- gg + annotation_custom(grob,
                       xmin = long - xdiff,
                       xmax = long + xdiff,
                       ymin = lat - ydiff,
                       ymax = lat + ydiff)
return(gg)

# add a bunch of the files to the  Map
Reduce(addmap,grobs,gg)

ggrepeleld flagmap (too much repel)

In this trial I chose values for the bounding boxes, padding, and force. All of these will have important effects on how the images/grobs are moved around and will need to be explored. I also uncovered what I believe to be an error whereby boxes can escape ggrepel’s bounding box.


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