title: Changing legend title ggplot
body: <p>I cannot seem to change the legend title without the legend splitting my shape and color into two separate legends. How can i change the combined legend title? The image is what the graph looks like.</p>
  
  <p><img src="https://i.stack.imgur.com/1REsZ.png" alt=""></p>
  
  <pre><code>ggplot(data = df, aes (x = factor(dminp,c("-3 to -1", "-1 to 1")), y = sum_diff,col = factor(dmin), shape = factor(dmin), group = factor(dmin)))+
  xlab("Range of Difficulty Parameters for Screen Items") + ylab("Bias Due to Skip-Logic") +
    stat_summary(geom = "point",fun.y = "mean",size = 8, aes(shape = factor(dmin)))+
    stat_summary(geom = "point",fun.y = "mean",size = 8, aes(col = factor(dmin)))+
   scale_shape_manual(values = c(8,5)) + theme_bw() + scale_colour_manual(values = c("orange","purple"))+
    theme(panel.grid.major.x  =   element_blank(),
          panel.grid.major =   element_line(colour = "black",size=0.25))+ theme(legend.justification = "top") 
  </code></pre>
  
  <p>I have tried using labs(col = "what i want it to be named") but this adds a 2nd legend and splits shape/color.</p>