Add argument for the number of y-axis grid lines

This allows the user to control how many y-axis grid lines are drawn,
instead of being restricted to the default of 5. Thanks to Fredrik
Wartenberg for the feature request and patch!

Fixes #374.

R/plot.R

@@ -135,6 +135,7 @@ plot.xts <- function(x,
                      yaxis.same=TRUE,
                      yaxis.left=TRUE,
                      yaxis.right=TRUE,
+                     yaxis.ticks=5,
                      major.ticks="auto",
                      minor.ticks=NULL,
                      grid.ticks.on="auto",
@@ -197,6 +198,7 @@ plot.xts <- function(x,
                     yaxis.same=yaxis.same,
                     yaxis.left=yaxis.left,
                     yaxis.right=yaxis.right,
+                    yaxis.ticks=yaxis.ticks,
                     major.ticks=major.ticks,
                     minor.ticks=minor.ticks,
                     grid.ticks.on=grid.ticks.on,
@@ -257,6 +259,7 @@ plot.xts <- function(x,
   cs$Env$theme$las <- if (hasArg("las")) eval.parent(plot.call$las) else 0
   cs$Env$theme$cex.axis <- if (hasArg("cex.axis")) eval.parent(plot.call$cex.axis) else 0.9
   cs$Env$format.labels <- format.labels
+  cs$Env$yaxis.ticks <- yaxis.ticks
   cs$Env$major.ticks <- if (isTRUE(major.ticks)) "auto" else major.ticks
   cs$Env$minor.ticks <- if (isTRUE(minor.ticks)) "auto" else minor.ticks
   cs$Env$grid.ticks.on <- if (isTRUE(grid.ticks.on)) "auto" else grid.ticks.on
@@ -1376,7 +1379,7 @@ new.replot_xts <- function(frame=1,asp=1,xlim=c(1,10),ylim=list(structure(c(1,10
 
   # function to plot the y-axis grid lines
   replot_env$Env$y_grid_lines <- function(ylim) {
-    p <- pretty(ylim, 5)
+    p <- pretty(ylim, Env$yaxis.ticks)
     p <- p[p >= ylim[1] & p <= ylim[2]]
     return(p)
   }

inst/unitTests/runit.plot.R

@@ -41,3 +41,14 @@ test.ylim_set_before_rendering <- function() {
   p <- plot(xts(x, .Date(1:10)))
   checkEqualsNumeric(range(x), p$get_ylim()[[2]])
 }
+
+test.yaxis.ticks <- function() {
+
+  x <- xts(rnorm(50), .Date(1:50))
+  ylim <- c(0, 10) # default case
+  p1 <- plot(x)
+  checkIdentical(pretty(ylim, 5), p1$Env$y_grid_lines(ylim))
+
+  p2 <- plot(x, yaxis.ticks = 10) # twice as many y-axis grid lines
+  checkIdentical(pretty(ylim, 10), p2$Env$y_grid_lines(ylim))
+}

man/plot.xts.Rd

@@ -9,7 +9,7 @@
   dn.col = NULL, bg = "#FFFFFF", type = "l", lty = 1, lwd = 2, lend = 1,
   main = deparse(substitute(x)), observation.based = FALSE,
   ylim = NULL, yaxis.same = TRUE, yaxis.left = TRUE, yaxis.right = TRUE,
-  major.ticks = "auto", minor.ticks = NULL,
+  yaxis.ticks = 5, major.ticks = "auto", minor.ticks = NULL,
   grid.ticks.on = "auto", grid.ticks.lwd = 1, grid.ticks.lty = 1,
   grid.col = "darkgray", labels.col = "#333333", format.labels = TRUE,
   grid2 = "#F5F5F5", legend.loc = NULL, extend.xaxis = FALSE)
@@ -62,6 +62,9 @@ separate panel.}
 
 \item{yaxis.right}{if TRUE, draws the y axis on the right}
 
+\item{yaxis.ticks}{desired number of y axis grid lines. The actual number
+  of grid lines is determined by the \code{n} argument to \code{\link{pretty}}.}
+
 \item{major.ticks}{period that specifies where tick marks and labels will be
   drawn on the x-axis. See Details for possible values.}