title: Change the number of tick marks on a figure in R
body: <p>I created a figure of two plots (two years) of climate data (temp and precip) that looks exactly like I want it, except that one of my axes has too many tick marks.  With everything I have going on with this figure, I can't find a way to specify fewer tick marks without messing up other parts.  I would also like to specify where the tick marks are.  Here is the figure: <img src="https://i.imgur.com/XyVeWlh.png" alt="Climate"></p>
  
  <p>You can see that the tick marks for the top axis just blur together and the numbers chosen are not very meaningful to me.  How can I tell R what I really want?</p>
  
  <p>Here are the datasets I am using:  <a href="https://www.dropbox.com/s/ysoiut7ma5y8q30/cobs10.txt" rel="noreferrer">cobs10</a> and
  <a href="https://www.dropbox.com/sh/phonq4ssjv5dnb0/7iLelar0pR" rel="noreferrer">cobs11</a>.</p>
  
  <p>And here is my code:</p>
  
  <pre><code>par(mfrow=c(2,1))
  par(mar = c(5,4,4,4) + 0.3)
  plot(cobs10$day, cobs10$temp, type="l", col="red", yaxt="n", xlab="", ylab="",     
  ylim=c(-25, 30))
  
  axis(side=3, col="black", at=cobs10$day, labels=cobs10$gdd) 
  at = axTicks(3)
  mtext("Thermal Units", side=3, las=0, line = 3)
  
  axis(side=2, col='red', labels=FALSE)
  at= axTicks(2)
  mtext(side=2, text= at, at = at, col = "red", line = 1, las=0)
  mtext("Temperature (C)", side=2, las=0, line=3)
  
  par(new=TRUE)
  plot(cobs10$gdd, cobs10$precip, type="h", col="blue", yaxt="n", xaxt="n", ylab="",     
  xlab="")
  axis(side=4, col='blue', labels=FALSE)
  at = axTicks(4)
  mtext(side = 4, text = at, at = at, col = "blue", line = 1,las=0)
  mtext("Precipitation (cm)", side=4, las=0, line = 3)
  
  
  par(mar = c(5,4,4,4) + 0.3)
  plot(cobs11$day, cobs11$temp, type="l", col="red", yaxt="n", xlab="Day of Year",     
  ylab="", ylim=c(-25, 30))
  
  axis(side=3, col="black", at=cobs11$day, labels=cobs11$gdd) 
  at = axTicks(3)
  mtext("", side=3, las=0, line = 3)
  
  axis(side=2, col='red', labels=FALSE)
  at= axTicks(2)
  mtext(side=2, text= at, at = at, col = "red", line = 1, las=0)
  mtext("Temperature (C)", side=2, las=0, line=3)
  
  par(new=TRUE)
  plot(cobs11$gdd, cobs11$precip, type="h", col="blue", yaxt="n", xaxt="n", ylab="",     
  xlab="", ylim=c(0,12))
  axis(side=4, col='blue', labels=FALSE)
  at = axTicks(4)
  mtext(side = 4, text = at, at = at, col = "blue", line = 1,las=0)
  mtext("Precipitation (cm)", side=4, las=0, line = 3)
  </code></pre>
  
  <p>Thanks for thinking about it.</p>