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    <p>Using a reproducible example</p> <pre><code>set.seed(1) x &lt;- as.POSIXct(sort(sample(100000, 100)), origin="1960-01-01") y &lt;- rpois(100, 5) plot(x, y, type = "l", xlab = "Time") </code></pre> <p>we can make use of the <code>axis.POSIXct()</code> function (one could also use the <code>Axis()</code> S3 generic as well) to add a custom axis to the plot. The main point here is that you, the user, are in full control of where ticks are draw and how they are labelled, you just need to work a little harder if the defaults don;t work for you.</p> <p>First we plot the data but suppress drawing of the x-axis:</p> <pre><code>plot(x, y, type = "l", xlab = "Time", xaxt = "n") </code></pre> <p>Next I will add a major tick mark at the start of each hour. For this I create a sequence of datetimes that goes</p> <ol> <li><strong>from</strong> the rounded hour of the first observation in the series,</li> <li><strong>to</strong> the end of the last hour in which an observation was made (using <code>ceiling()</code> to move us on to the next hour),</li> <li><strong>incrementing</strong> the sequence in 1 hour (<code>by = "1 hour"</code>) units.</li> </ol> <p>This sequence is supplied to the the <code>at</code> argument of <code>axis.POSIXct()</code>. The rest should be easy to follow if not read <code>?axis.POSIXct</code> and <code>?par</code></p> <pre><code>## add axis tick at each hour: axis.POSIXct(side = 1, x = x, at = seq(from = round(x[1], "hours"), to = x[1] + ceiling(difftime(tail(x, 1), head(x, 1), units = "hours")), by = "1 hour"), las = 2) </code></pre> <p>The resulting figure looks like this:</p> <p><img src="https://i.stack.imgur.com/eunaZ.png" alt="enter image description here"></p> <p>To show finer control, I now add minor tick marks at each half-hour location, but suppress the annotation of those ticks (via the <code>labels</code> argument) and also make the minor ticks shorter (via graphics parameter <code>tcl</code>). Notice how the <code>seq()</code> method's <code>by</code> argument can take a numeric amount of the stated interval</p> <pre><code>## add minor ticks at 30 min intervals to above plot axis.POSIXct(side = 1, x = x, at = seq(from = round(x[1], "hours"), to = x[1] + ceiling(difftime(tail(x, 1), head(x, 1), units = "hours")), by = "30 mins"), las = 2, tcl = -0.2, labels = FALSE) </code></pre> <p>The plot now looks like this:</p> <p><img src="https://i.stack.imgur.com/8Twh2.png" alt="enter image description here"></p> <p>You can add your own labels rather than the ones that the <code>axis.POSIXct</code> function comes up with. If you want to do this, then we should assign the output from <code>seq()</code> to an object that we can then use the <code>format()</code> function on. For example:</p> <pre><code>plot(x, y, type = "l", xlab = "Time", xaxt = "n") tseq &lt;- seq(from = round(x[1], "hours"), to = x[1] + ceiling(difftime(tail(x, 1), head(x, 1), units = "hours")), by = "1 hour") axis.POSIXct(side = 1, x = x, at = tseq, labels = format(tseq, format = "%H:%M"), las = 2) </code></pre> <p>The resulting plot is shown below:</p> <p><img src="https://i.stack.imgur.com/6EDtH.png" alt="enter image description here"></p> <p><code>format()</code> returns a character string of the formatted datetime. You can <code>paste()</code> on anything else you want or look at the other placeholders that can be used to format datetime objects in <code>?strftime</code></p>
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    1. POR Plot Specify number of time tickmarks - time/date equivalent to pretty
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    1. USGavin Simpson
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    1. COSorry for the effort that went into this answer, but you are still picking hours or half hours yourself. What would happen if there only were 10 seconds between start and end time? To give you an example, in my particular case I have an interactive plot - using `getGraphicsEvent()` - where I can zoom into the plot, and the further I zoom with the mouse, the plot axis slowly needs to switch from hours to minutes to seconds to milliseconds - and back, once I zoom back out.
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    2. COI only chose those time intervals because you *explicitly* mentioned them. There is nothing in R canned that I am aware of that will do what you want so you'll need to combine the custom axis approach I show with a hacked version of `axis.POSIXct()`. See my other Answer for pointers on that. (Coming soon)
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    3. COI am not disputing that it is possible to come up with a function in R (after all, R is just C and C++, and allows more third party plugins, so virtually anything is possible), all I am saying that so far, none is out there, and I haven't found the time to write one myself.
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