title: add confidence interval to splines from quantile regression
body: <p>I am using quantreg to fit a quantile regression and have also added some knots based on x-values (quantiles). I now would like to plot this and also have the confidence intervals. Not sure how to do that.</p>
  
  <p>Here is a reproducible example:</p>
  
  <pre><code>  #create data    
    x &lt;- seq(0,100,length.out = 100)        
    sig &lt;- 0.1 + 0.05*x 
    b_0 &lt;- 6                                
    b_1 &lt;- 0.1                              
   set.seed(1)                             
   e &lt;- rnorm(100,mean = 0, sd = sig)      
   y &lt;- b_0 + b_1*x + e 
  
  mydata &lt;- data.frame(x,y, age=sample(30:70,100,replace=TRUE), sex=sample(c("Male","Female"),100, replace=TRUE))
  
  #run regression
  library(quantreg)
  library(splines)
  model &lt;- rq(y ~ ns(x, knots=c(25,50,75))+age+sex, tau=0.5, data=mydata ) 
  
  #plot
  sp &lt;- c(25,50,75)
  ggplot(mydata, aes(x=x,y=y))+ geom_point()+ geom_quantile(formula=y~ns(x,knots=sp), quantiles=0.5, se=T)
  </code></pre>
  
  <p>This does not show the confidence intervals??
  Also, this plot does not take the covariates into account? Is there a way to do this?</p>