[R] Adding Year-Month-Day to X axis

[Gregory Coats]

Since the horizontal axis side=1 is year-month-day, how do I issue an abline command to draw dashed vertical lines, as a background grid, within the graph’s border? Similar to the abline command I call below, in blue, for dashed horizontal lines, as a background grid.
Greg

y_duration <- c (301.59050, 387.35700, 365.64366, 317.26150, 321.71883, 342.44950, 318.95350, 322.33233, 330.60333, 428.99516, 297.82066, 258.23166, 282.01816)
x_yyyymmdd <-as.Date(c ("2018-04-25", "2018-04-26", "2018-04-27", "2018-04-28", "2018-04-29", "2018-04-30", "2018-05-01", "2018-05-02", "2018-05-03", "2018-05-04", "2018-05-05", "2018-05-06", "2018-05-07"), format="%Y-%m-%d")
par     (mar=c(6,4,4,2))
plot    (x_yyyymmdd, y_duration, type="l", xaxt="n", yaxt="n", ylim=range(240,480), xlab="", ylab="", col="blue")
abline  (h=c(240,270,300,330,360,390,420,450,480,510,540), lty=3, lwd=1.0, col="grey50")
axis    (side=2, at=240, cex.axis=1.0, label="4:00")
axis    (side=2, at=300, cex.axis=1.0, label="5:00")
axis    (side=2, at=360, cex.axis=1.0, label="6:00")
axis    (side=2, at=420, cex.axis=1.0, label="7:00")
axis    (side=2, at=480, cex.axis=1.0, label="8:00")
axis    (side=1, at=x_yyyymmdd, labels=format(x_yyyymmdd, "%Y-%m-%d"), las=2)


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