title: Plot line graphs with with a factor variable on x-axis
body: <p>I failed to plot a line graph on x axis using initials of months with this code:</p>
  
  <pre><code>yrange&lt;-range(c(Estimate, lcl,ucl))
  plot(nmonth, Estimate, type = "b", pch = 20, ylim = yrange, 
  xlab = "Months",  ylab = expression(hat(beta) * " estimates" * " &amp; " * "   confidence "  * " levels "))
  lines(nmonth, ucl, lty = 2)
  lines(nmonth, lcl, lty = 2)
  abline(h = 0, lty = 3)
  </code></pre>
  
  <p>and with this as well.</p>
  
  <pre><code>ggplot(data=df1, aes(x=nmonth, y=Estimate)) + geom_line() + geom_point() + geom_line(size=0.1) + geom_line(aes(y = ucl)) + geom_line(aes(y = lcl))
  </code></pre>
  
  <p>Using numeric month(nmonth) I can produce a plot, but labels are not what I wished to have.
  How can I plot with all initials of the months on x axis? </p>
  
  <p>The data is this one:</p>
  
  <pre><code>structure(list(Estimate = c(0.00571942142644563, 0.0111649330056159, 
  0.0143761435860972, 0.00739757934210567, 0.00110764672100624, 
  0.00168566337236168, 0.00392476757483504, 0.00234423892025447, 
  0.000166724737089459, -0.0014580012873366, -0.00197786373686253, 
  -0.00216289530501664), se = c(0.004018593736177, 0.0040534199847734, 
  0.0041113846550833, 0.00402501059422328, 0.00393358629717884, 
  0.00370406599461686, 0.003796651550619, 0.00392460643968604, 
  0.00376380927915926, 0.00391408378704714, 0.00388845564349082, 
  0.00394365265230613), nmonth = 1:12, month = structure(c(1L, 
  2L, 3L, 4L, 3L, 1L, 1L, 4L, 5L, 6L, 7L, 8L), .Label = c("J", 
  "F", "M", "A", "S", "O", "N", "D"), class = "factor"), lcl = c(-0.00215702229646129, 
  0.00322022983546004, 0.00631782966213393, -0.000491441422571959, 
  -0.00660218242146429, -0.00557430597708737, -0.0035166694643782, 
  -0.00534798970153017, -0.00721034145006269, -0.00912960550994899, 
  -0.00959923679810454, -0.00989245450353666), ucl = c(0.0135958651493525, 
  0.0191096361757718, 0.0224344575100605, 0.0152866001067833, 0.00881747586347677, 
  0.00894563272181073, 0.0113662046140483, 0.0100364675420391, 
  0.00754379092424161, 0.00621360293527579, 0.00564350932437948, 
  0.00556666389350337)), .Names = c("Estimate", "se", "nmonth", 
  "month", "lcl", "ucl"), class = "data.frame", row.names = c(NA, 
  -12L))
  </code></pre>