title: Plot columns from list of data frames
body: <p>I'm trying to subset some dates into season then combine monthly plots of 'date and amplitude' for each season.</p>
  
  <pre><code>head(dat)
      Date  Amplitude
  1 2009-09-13 -2.6966667
  2 2009-09-14 -0.7264583
  3 2009-09-15 -2.1823333
  </code></pre>
  
  <p>Here's my code to create seasons and YearMonth for plot titles:</p>
  
  <pre><code>x &lt;- as.Date(dat$Date, format="%Y/%m/%d")
  x &lt;- as.Date(x, origin="2009-09-13")
  x &lt;- cut(x, breaks="quarter")
  labs &lt;- paste(substr(levels(x),1,4),"/",1:4, sep="")
  dat$DateQ &lt;- factor(x, labels=labs)
  dat$YearMonth &lt;- format(dat$Date, "%Y/%m")
  head(dat)
  Date  Amplitude  DateQ YearMonth
  1 2009-09-13 -2.6966667 2009/1   2009/09
  2 2009-09-14 -0.7264583 2009/1   2009/09
  </code></pre>
  
  <p>I used a split to create data frames for each season and then named the data frames with a for loop</p>
  
  <pre><code>dat_split_season &lt;- split(dat, dat$DateQ)
  new_names &lt;- c("Summer_2009", "Fall_2009", "Winter_2010", "Spring_2010", 
            "Summer_2010", "Fall_2010", "Winter_2011",
            "Spring_2011", "Summer_2011", "Fall_2011",
            "Winter_2012", "Spring_2012", "Summer_2012")
  for (i in 1:length(dat_split_season)) {
  assign(new_names[i], dat_split_season[[i]])
  }
  </code></pre>
  
  <p>I can create the desired monthly plots by season manually by changing df</p>
  
  <pre><code>df &lt;- Winter_2012
  graphs &lt;- lapply(split(df,df$YearMonth),
     function(gg) ggplot(gg, aes(x=Date, y=Amplitude)) +
       geom_point() + geom_line() + ylim(-10, 10) +
       ggtitle(paste(gg$YearMonth)) + theme_bw())
  do.call("grid.arrange", graphs)
  </code></pre>
  
  <p>I would like to use a for loop or the apply() family to sequence through the seasons list, selecting the Date and Amplitude columns for each data frame and plotting by 'YearMonth'.</p>
  
  <p>The issue I'm having is probably syntax related; I'm new to R and programming in general. But I have spent the last few days scouring the forum and I would appreciate some direction at this point. Thanks!</p>