Refactor add_TA to use y_grid_lines instead of custom function

The original custom grid_lines function is very similar to y_grid_lines
in replot_xts object but the latter restricts the output values to
the upper limit and lower limit. About two intervals will be
dropped so the labels of prior chart will not be blocked.

Author: erichung0404

R/chart_Series.R

@@ -522,10 +522,8 @@ add_TA <- function(x, order=NULL, on=NA, legend="auto",
 
     plot_object$add_frame(ylim=range(na.omit(xdata)),asp=1)  # need to have a value set for ylim
     plot_object$next_frame()
-  # add grid lines, using custom function for MACD gridlines
-  lenv$grid_lines <- function(xdata,xsubset) { 
-    pretty(xdata[xsubset])
-  }
+  # add grid lines, using y_grid_lines function from replot_xts object
+  lenv$grid_lines <- plot_object$Env$y_grid_lines
   exp <- c(expression(segments(1,grid_lines(xdata,xsubset),NROW(xdata[xsubset]),grid_lines(xdata,xsubset),
                                col=theme$grid)), exp,  # NOTE 'exp' was defined earlier to be plot_macd
   # add axis labels/boxes