title: R Using ggplot with a list of dataframes
body: <p>I have a list of dataframes (df_list) which I want to plot with ggplot. I want to display the Date on the x-scale (its always the same of course) and then 2 different geo_lines. The first geoline out of the df1-3 and the 2nd line from abc. Here is an examle code:</p>
  
  <pre><code>library(lubridate)
  v1 = seq(ymd('2000-05-01'),ymd('2000-05-10'),by='day')
  v2 = seq(2,20, length = 10)
  v3 = seq(-2,7, length = 10)
  v4 = seq(-6,3, length = 10)
  df1 = data.frame(Date = v1, df1_Tmax = v2, df1_Tmean = v3, df1_Tmin = v4)
  
  v1 = seq(ymd('2000-05-01'),ymd('2000-05-10'),by='day')
  v2 = seq(3,21, length = 10)
  v3 = seq(-3,8, length = 10)
  v4 = seq(-7,4, length = 10)
  df2 = data.frame(Date = v1, df2_Tmax = v2, df2_Tmean = v3, df2_Tmin = v4)
  
  v1 = seq(ymd('2000-05-01'),ymd('2000-05-10'),by='day')
  v2 = seq(4,22, length = 10)
  v3 = seq(-4,9, length = 10)
  v4 = seq(-8,5, length = 10)
  df3 = data.frame(Date = v1, df3_Tmax = v2, df3_Tmean = v3, df3_Tmin = v4)
  
  v1 = seq(ymd('2000-05-01'),ymd('2000-05-10'),by='day')
  v2 = seq(2,20, length = 10)
  v3 = seq(-2,8, length = 10)
  v4 = seq(-6,3, length = 10)
  abc = data.frame(Date = v1, ABC_Tmax = v2, ABC_Tmean = v3, ABC_Tmin = v4)
  
  df_list = list(df1, df2, df3, abc)
  names(df_list) = c("df1", "df2", "df3", "abc")
  </code></pre>
  
  <p>For example I want to plot <code>df_list$df1$df1_Tmax</code> together with <code>df_list$abc$ABC_Tmax</code>. </p>
  
  <p>I tried this:</p>
  
  <pre><code>ggplot(data = df_list, aes(x = df1$Date)) +
    geom_line(aes(y = df1$df1_Tmax), color = "darkgreen", size = 1) +
    geom_line(aes(y = abc$ABC_Tmax), color = "grey27", size = 1) +
    labs(title="Title", 
         subtitle="subtitle", 
         x = "day", 
         y = "T") +
    scale_x_date(date_labels="%d",date_breaks  ="1 day")
  </code></pre>
  
  <p>But I get this error message:</p>
  
  <pre><code>Error: `data` must be a data frame, or other object coercible by `fortify()`, not a list
  </code></pre>
  
  <p>Any ideas how to solve this problem?</p>