title: How to set the length of ticks to be exactly 1 inch?
body: <pre><code> x&lt;-seq(-3,3,by=0.1)
   y=0.5*x^3 
   ## Set png size to be slightly bigger than plot to take account of one line margin
   png( "~/myplot.png" , width = 600 , height = 2600 ,units="px",res=100)
   ## Set graphical parameters to control the size of the plot area and the margins
   ## Set size of plot area to 6 inches wide by 15 inches tall
   par( pin = c(6,26) )
   ## Remove margins around plot area
   par( mai = c(0,0,0,0) )
   ## Remove outer margins around plot area
   par( omi = c(0,0,0,0) )
   ## Set y-axis take use entire width of plot area (6 inches) and to have 7 tick marks (-3,-2,-1,0,1,2,3)
   par( xaxp = c(-3,3,7) )
   ## Set x-axis take use entire height of plot area (26 inches) and to have 27 tick marks (-13,-12,...,0,...11,12,13)
   par( yaxp = c(-13,13,27) )
   ## Create the plot
   plot( x , y , type = "l" , frame.plot = FALSE , axes = FALSE )
   axis( 1 , pos = 0 , at = seq( -3 , 3 , by = 1 ) , labels = seq( -3 , 3 , by = 1 ) )
   axis( 2 , pos = 0 , at = seq( -13 , 13 , by = 1 ) , labels = seq( -13 , 13 , by = 1 ) )
   text(0.5,5,expression(f(x)==frac(1,2)*x^3) )
   ## Turn PNG device off
   dev.off()
  </code></pre>
  
  <p>We can get the graph which is 6 inches width, 26 inches height, but the tick unit in the x-axis or y-axis is not 1 inch; please see the attachment g1.png:</p>
  
  <p><img src="https://i.stack.imgur.com/7G0bP.png" alt="enter image description here"></p>
  
  <p>The reason is R will not use the entire space to set the x-axis and y-axis, R keep little space; please see the attachment g2.png:</p>
  
  <p><img src="https://i.stack.imgur.com/ti7DW.png" alt="enter image description here"></p>
  
  <p>I can <code>png( "~/myplot.png" , width = 610 , height = 2700 ,units="px",res=100)</code>, but how can I make the physical length of tick unit just to be 1 inch?</p>