title: Removing unwanted characters from regression line equation
body: <p>In prior builds of R/R-Studio I've used, when applying a regression formula to a ggplot, I would get a graph with the regression equation properly rendered. However, now that I've switched to R v3.5.3, I'm getting extra characters in the regression line. I've modified a prior question (<a href="https://stackoverflow.com/questions/49418552/adding-regression-line-equation-and-r2-on-separate-lines-graph">Adding Regression Line Equation and R2 on SEPARATE LINES graph</a>) as an example:</p>
  
  <pre><code>library(ggplot2)
  set.seed(5)
  df &lt;- data.frame(x = c(1:50))
  df$y &lt;- df$x + rnorm(50, sd=5)
  
  lm_eqn &lt;- function(df){
    m &lt;- lm(y~x, df)
    eq &lt;- substitute(italic(hat(y)) == a + b %.% italic(x)*","~~italic(r)^2~"="~r2,
                     list(a = format(coef(m)[1], digits=3),
                          b = format(coef(m)[2], digits=3),
                          r2 = format(summary(m)$r.squared, digits=3)))
    as.character(as.expression(eq))}
  
  ggplot(data=df, aes(x=x, y=y))+
    geom_smooth(method="lm", se=FALSE, color="black", formula=y~x)+
    geom_point()+
    geom_text(x=10, y=50, label=lm_eqn(df), parse=TRUE)
  </code></pre>
  
  <p>I expect the regression line text to be </p>
  
  <blockquote>
    <p>y^=-0.162+1.02·x, r²=0.886</p>
  </blockquote>
  
  <p>However, what shows up is  </p>
  
  <blockquote>
    <p>y^=c(-0.162)+c(1.02)·x, r²=0.886</p>
  </blockquote>
  
  <p>Is there a way to remove the <code>c</code> and <code>()</code>, which did not show in the previous ggplot, or is this a bug?</p>