r - ggplot2: Why are my text annotations not aligned right? -

i trying label straight line segment using annotate. however, setting angle of annotation slope of line not align annotation line segment.

library(ggplot2)  ggplot(data.frame(x = seq(0, 14, 0.1)), aes(x = x)) +    stat_function(fun = function(x) {     14 - x   }, geom = "line") +    theme_bw() +    annotate(     geom = "text",      x = 7.5, y = 7.5,     label = "x + y = 14",     angle = -45)  # nistunits::nistradiantodeg(atan(-1)) 

this gives:

can explain phenomenon, , how fix this, i.e., align annotation have same angle line segment?

i believe should give want. see answer reference get width of plot area in ggplot2.

#plot can reference viewport ggplot(data.frame(x = seq(0, 14, 0.1)), aes(x = x)) +    stat_function(fun = function(x) {     14 - x   }, geom = "line")  #get currentvptree , convert vp's height/width inches current.vptree() <- convertwidth(unit(1,'npc'), 'inch', true) b <- convertheight(unit(1,'npc'), 'inch', true)  #replot using ratio of a/b appropriate angle ggplot(data.frame(x = seq(0, 14, 0.1)), aes(x = x)) +    stat_function(fun = function(x) {     14 - x   }, geom = "line") +    theme_bw()+    annotate(     geom = "text",      x = 7.5, y = 7.5,     label = "x + y = 14",     angle = (atan(a/b) * 180/pi) + 270)  

we viewport width/height , use simple geometry (inverse tangent since have both sides of triangle) calculate actual angle of line is.

result: