title: Subset Data for ggplot2 graph
body: <p>I am working with ggplot2 and have a question about how to subset data for plots.
  I have the following dataset (example) and need to create a line plot comparing Q1 data by year of Company A. </p>
  
  <p>x= 2015 Q1, 2016 Q1, 2017 Q1
  y= Data for Company A</p>
  
  <pre><code>Company Year    Quarter Data
  A       2015    Q1  1
  B       2015    Q1  2
  C       2015    Q1  3
  A       2015    Q2  4
  B       2015    Q2  5
  C       2015    Q2  6
  A       2015    Q3  7
  B       2015    Q3  8
  C       2015    Q3  9
  A       2016    Q1  10
  B       2016    Q1  11
  C       2016    Q1  12
  A       2016    Q2  13
  B       2016    Q2  14
  C       2016    Q2  15
  A       2016    Q3  17
  B       2016    Q3  18
  C       2016    Q3  19
  </code></pre>
  
  <p>For other graphs involved in this project I've been using this code:</p>
  
  <pre><code>ggplot(df[df$Company=="A",], aes(x=   , y=   , group=1)) +
    geom_line(color='steelblue', size=2) + geom_point(aes(color=Company))+
    xlab("Q1 by Year") +
    ylab("Data") + theme_minimal(base_size=12)+
    ggtitle("  ")+
    theme(plot.title=element_text(hjust=0.5, size=16, face="bold"))+
    theme(axis.text.x=element_text(size=10, vjust=0.5, color="black", face="bold"),
          axis.text.y=element_text(size=10, vjust=0.5, color="black", face="bold"),
          axis.title.x=element_text(size=13, face="bold"),
          axis.title.y=element_text(size=13, face="bold"))+
    theme(aspect.ratio=3/4) + scale_color_brewer(palette="Set2") + 
    theme(legend.position="none")
  </code></pre>
  
  <p>Any suggestions on how to subset this data for my needed graph? This is one of the things I struggle with the most. Any help would be appreciated! Thank you!</p>