[R] "lwd" gives different plotted line widths for Vertical and horizontal Lines when lwd= 2?

[hadley wickham]

On Sun, Jul 6, 2008 at 6:33 AM, Pedro de Barros <pbarros at ualg.pt> wrote:
> Dear All,
>
> I have been trying to add lines to the axis grobs of plots produced with
> ggplot2.
>
> The code I have used is below. It works, although I do not think it is a
> really elegant way of doing what I want....
> However, I am now noticing that when plotted, the width of the lines in the
> axis are not the same as the line widths I get for the ticks (the ticks get
> thicker lines).  Also, when I build a function to do the same conversion, I
> get different (thinner) line widths for the x-axis I have been trying to
> find out why this is so, but I seem to have met a dead-end.
>
> #Test
> library(ggplot2)
> #Set the basic overall plot options
> themePublish <-list(background.fill='white', grid.fill='white',
> grid.colour='NA', grid.minor.colour='NA', axis.colour='black',
> border.colour='NA')
> ggtheme(themePublish)
>
> #Generate the data
> plotdata<-data.frame(x=1:10, y=runif(10))
>
> #Build the ggplot object
>
> ggPlotObject<-ggplot()
> ggPlotObject<-ggPlotObject+layer(data=plotdata,
> mapping=aes_string(x='x',y='y'),geom='point', stat='identity', size=4)
> ggPlotObject<-ggPlotObject+ scale_y_continuous(limits=c(0,1), expand=c(0,0))
>
> #################
> #I define the following function
> F.ggPlot.AddAxisLines<-function(ggPlotObject, col='black', lwd=2,
> lty='solid'){
> ###################################################################################################
> # First, capture the ggPlot object as a gTree
> ggPlotTree<-ggplot_plot(ggPlotObject)
>
> #Get the parameters defined for the lines
> LinePars <- gpar(col=col, lwd=lwd, lty=lty)
>
> #Then, create the objects to add to the axes
> YAxis.Line<-linesGrob(x=unit(c(1,1), 'npc'), y=unit(c(0,1), 'npc'),
> vp='left_axis', name='YAxisLine', gp=LinePars)
> XAxis.Line<-linesGrob(x=unit(c(0,1), 'npc'), y=unit(c(1,1), 'npc'),
> vp='bottom_axis', name='XAxisLine', gp=LinePars)
>
> #Add the grobs to this object
> ggPlotTreeMod<-ggPlotTree
> ggPlotTreeMod<-addGrob(ggPlotTreeMod, YAxis.Line, gPath('yaxis'), grep=TRUE)
> ggPlotTreeMod<-addGrob(ggPlotTreeMod, XAxis.Line, gPath('xaxis'), grep=TRUE)
>
> #Edit the properties of the ticks, to match those of the lines
> ggPlotTreeMod<-editGrob(ggPlotTreeMod, gPath('ticks'), gp=LinePars,
> grep=TRUE, global=TRUE)
>
> #Output the new object
>
> ggPlotTreeMod
> }
>
> # and then do
> ggPlotTreeMod2 <- F.ggPlot.AddAxisLines(ggPlotObject)
> grid.draw(ggPlotTreeMod2)
>
> # I get different line thicknesses for the two axes
>
> #However, if I do
> ggPlotTreeMod3 <- F.ggPlot.AddAxisLines(ggPlotObject, lwd=3)
> grid.draw(ggPlotTreeMod3)
>
> # I get the same line thickness on both axes, but thicker ticks

How are you judging thickness?  It may be that due to the size of
ticks and the size of the plot surface that thickness of the line is
not a whole number of pixels, and depending on the exact position of
the line, gets rounded up or down.  Trying saving as pdf and zooming
in.

Hadley

-- 
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