使用 par 设置 png 绘图设备的绘图边距

Question

I've created a choropleth of Brazil. When saving the plot in .png, the upper and the lower part of the plot are lost (covered). Here are the lines to save the plot.

plot.new()
par(omi=c(0,0,0,0), mgp=c(0,0,0),mar=c(0,0,0,0) , family = "D")
par(mfrow=c(1,1),cex=1,cex.lab = 0.75,cex.main=0.2,cex.axis=0.2)
png(filename = "map_cons_g.png", width = 6,height = 6, units = "in", res = 600)
plot(c(-75,-35),c(0,-30),type="n",axes=FALSE,xlab="",ylab="",asp=1.2)
plot(Brazil,col=cols[Brazil$Cons.g_ri],add=TRUE,border="black",lwd=0.5)
dev.off()

For saving the plot without losing the upper and the lower part of the map, I must change the coordinates to add white space at the bottom and at the top (ie replace c(0,-30) by c(5,-33)):

plot.new()
par(omi=c(0,0,0,0), mgp=c(0,0,0),mar=c(0,0,0,0) , family = "D")
par(mfrow=c(1,1),cex=1,cex.lab = 0.75,cex.main=0.2,cex.axis=0.2)
png(filename = "map_cons_g.png", width = 6,height = 6, units = "in", res = 600)
plot(c(-75,-35),c(5,-33),type="n",axes=FALSE,xlab="",ylab="",asp=1.2)
plot(Brazil,col=cols[Brazil$Cons.g_ri],add=TRUE,border="black",lwd=0.5)
dev.off()

This works in the sense that I can see the full map but the map then does not use all the available area in the figure. It seems that there are some margin in the upper and the lower part of the figure when saving the plot. I've never had that problem with other types of plot.

Sorry, I don't have enough "reputation" to post images to show you how the maps look like.

Any idea of how to fix this?

Edit:

The comments below got me searching more into the problem and I finally found a fix. I apologize as I now realized that I did not understand the source of the problem and thus did not explain as best as I could have,

It seems that png resets the outer margin of the plot. Thus, even though I had set omi=c(0,0,0,0), those were not the value used by the png command in saving the plot. The solution was to set the plot parameters after calling png so save the figure.

plot.new()
png(filename = "map_cons_g.png", width = 6,height = 6, units = "in", res = 600)
par(omi=c(0,0,0,0), mgp=c(0,0,0),mar=c(0,0,0,0) , family = "D")
par(mfrow=c(1,1),cex=1,cex.lab = 0.75,cex.main=0.2,cex.axis=0.2)
plot(c(-75,-35),c(5,-33),type="n",axes=FALSE,xlab="",ylab="",asp=1.2)
plot(Brazil,col=cols[Brazil$Cons.g_ri],add=TRUE,border="black",lwd=0.5)
dev.off()

Answer 1

From Details in ?par :

Each device has its own set of graphical parameters.

Thus, even though I had set the outer margin of the plot in par ( omi = c(0,0,0,0) ), those value were overwritten by the parameters in png when saving the plot.

The solution was to set the margin parameters in par after calling png

plot.new()

# first open png device...
png(filename = "map_cons_g.png", width = 6,height = 6, units = "in", res = 600)

# ...then set par
par(omi = c(0,0,0,0), mgp = c(0,0,0), mar = c(0,0,0,0), family = "D")
par(mfrow = c(1, 1), cex = 1, cex.lab = 0.75, cex.main = 0.2, cex.axis = 0.2)

plot(c(-75, -35), c(5, -33),  type = "n", axes = FALSE, xlab = "", ylab = "", asp = 1.2)
plot(Brazil, col = cols[Brazil$Cons.g_ri], add = TRUE, border = "black", lwd = 0.5)
dev.off()