check for ylim of 0s

- calling lines, points, or more generally addSeries with an xts
  object of all 0s results in an invalid ylim of (0,0)
- this adds a check for a ylim of (0,0) and if true sets the ylim
  to c(-1,1)
- see #164

R/plot.R

@@ -699,6 +699,7 @@ addSeries <- function(x, main="", on=NA, type="l", col=NULL, lty=1, lwd=1, pch=0
     ylim <- lenv$ylim  # lenv$ylim assigned via mapply above
   } else {
     ylim <- range(lenv$xdata[xsubset], na.rm=TRUE)
+    if(all(ylim == 0)) ylim <- c(-1, 1)
     lenv$ylim <- ylim
   }