title: Assign different continuous color gradients based on two different variables in ggplot2 in x and y direction
body: <p>I'm new to R (and stack overflow) and would appreciate any help! I haven't been able to find a solution from <a href="https://stackoverflow.com/questions/31706390/varying-gradient-using-ggplot2-in-r">other</a> questions. I'm trying to:</p>
  
  <ol>
  <li>Assign 2 different continuous color gradients in the x and y direction based on different variables. (plot1 in code below)</li>
  <li>Retain these color assignments for a different plot of different variables of the same data frame. (plot2 in code below)</li>
  </ol>
  
  <p>I have only been able to manage assigning color based on one variable as shown below. </p>
  
  <p>Example:</p>
  
  <pre><code>####set up data####
  #using ggplot2 package
  library(ggplot2)
  
  df &lt;- data.frame(c(1, 1, 1, 1, 2, 2, 2, 2, 3, 3, 3, 3, 4, 4, 4, 4), 
                   c(1, 2, 3, 4, 1, 2, 3, 4, 1, 2, 3, 4, 1, 2, 3, 4),
                   c(256, 4, 196, 16, 144, 36, 100, 64, 81, 49, 121, 25, 169, 9, 225, 1),
                   c(0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610))
  colnames(df) &lt;- c("X", "Y", "V1", "V2")
  
  ####plot1: Simple plot of V1 vs V2#####
   #I would like the color to go towards blue as V1 increases on x axis (as it is now)
   #I would also like the color to go towards red as V2 increases on y axis
   #Ideally, when both x and y are large, it would be some purple color
  
  plot1 &lt;- ggplot(df, aes(x=V1, y = V2, color= V1)) + geom_point()+ 
    scale_color_continuous(low="black", high="light blue")+ 
    labs(y= "V2: I want this to go from black --&gt; red", x = "V1: black--&gt; blue")
  plot1
  
  ####plot2: plot of x vs y but with color assignment same as plot1####
  #The position based on X and Y is correct but,
  #   I'd like to have the color assigned based on V1 and V2 (same as plot1)
  plot2 &lt;- ggplot(df, aes(x=X, y=Y, color=V1)) + geom_point()+
    scale_color_continuous(low="black", high="light blue")
  plot2
  </code></pre>