Align text inside a plot

While legend() is of course appropriate for legends, there is a general solution for all text. The trick is that the pos option not only sets the position of the text relative to the current location but it also sets justification. Above and Below are center justified. Setting pos to 2 makes the text right justified. When it is set to the right of the position (pos = 4) then it is left justified.

For left justified replace your text code with...

text(1.5, 150, paste("Mean =", round(MyMean, 1), "\nMedian =", 
         round(MyMedian, 1), "\nStd.Dev =", round(MySd, 1)), pos = 4)

and for right justified...

text(5.0, 150, paste("Mean = ", round(MyMean, 1), "\nMedian = ", 
        round(MyMedian, 1), "\nStd.Dev = ", round(MySd, 1), sep = ''), pos = 2)

Try using legend() instead of text()

legend(3.5, 150, legend = c(paste("Mean =", round(MyMean, 1)),
                            paste("Median =",round(MyMedian, 1)),
                            paste("Std.Dev =", round(MySd, 1))), 
                  bty = "n")

You'll have to play around with the position adjustment. You might consider not using xy coordinates at all, but replacing those two arguments with "topleft"