Map coordinates to actual pixel locations on a PNG device

Jason emailed me a new tip. Enjoy it!

Use grconvertX and grconvertY to map the X,Y coordinates for an entity on a graphics device to user coordinates. For example if you plotted points to an image and wanted to map those X,Y coordinates to the actual pixel locations on the PNG you would use this family of functions.

#
# Sample R Code for grconvertX and grconvertY
#

# make fake data
tDat <- cbind(rnorm(10), rnorm(10));

#
# Example #1 -- plot them to an X11 window
#
x11();
plot(tDat);
print(paste(grconvertX(tDat[, 1], "user", "device"), grconvertY(tDat[, 2], "user", "device")));

# turn off the x11 device
#dev.off()

#
# Example 2-- Get the pixel coordinates of the data on a PNG image
#
# plot to a PNG
png(file="RTip_coordinates.png", height=1000, width=1000);
plot(tDat);
print( paste(grconvertX(tDat[, 1], "user", "device"), grconvertY(tDat[, 2], "user", "device")));
dev.off()

# Now, go into GIMP or photoshop. At each data point should be at the
# X,Y coordinate listed.