Clean up plot_lines() in addPolygon()

This makes the function consistent with the similar function in
addSeries(). 'xDataSubset' prevents unnecessary repeated subsetting.
The 'xsubset' if statement handles some special cases. Merge with an
empty xts object to create 'ta.y' to avoid allocating the coredata.

R/plot.R

@@ -1023,20 +1023,27 @@ addPolygon <- function(x, y=NULL, main="", on=NA, col=NULL, ...){
   lenv$plot_lines <- function(x, ta, on, col, ...){
     xdata <- x$Env$xdata
     xsubset <- x$Env$xsubset
+    xDataSubset <- xdata[xsubset]
     if(is.null(col)) col <- x$Env$theme$col
     if(all(is.na(on))){
       # Add x-axis grid lines
-      x$Env$x_grid_lines(xdata[xsubset], x$Env$grid.ticks.on, par("usr")[3:4])
+      x$Env$x_grid_lines(xDataSubset, x$Env$grid.ticks.on, par("usr")[3:4])
     }
     # we can add points that are not necessarily at the points
-    # on the main series
-    subset.range <- paste(start(xdata[xsubset]),
-                          end(xdata[xsubset]),sep="/")
-    ta.adj <- merge(n=.xts(1:NROW(xdata[xsubset]),
-                           .index(xdata[xsubset]), 
-                           tzone=tzone(xdata)),ta)[subset.range]
+    # on the main series, but need to ensure the new series only
+    # has index values within the xdata subset
+    if(xsubset == "") {
+      subset.range <- xsubset
+    } else {
+      fmt <- "%Y-%m-%d %H:%M:%OS6"
+      subset.range <- paste(format(start(xDataSubset), fmt),
+                            format(end(xDataSubset), fmt), sep = "/")
+    }
+
+    xds <- .xts(, .index(xDataSubset), tzone=tzone(xdata))
+    ta.y <- merge(ta, xds)[subset.range]
     # NAs in the coordinates break the polygon which is not the behavior we want
-    ta.y <- na.omit(ta.adj[,-1])
+    ta.y <- na.omit(ta.y)
 
     # x coordinates
     n <- seq_len(NROW(ta.y))