How can I fix missing color scale in the legend when I plot continuous color using ggplot2?

Question

Using Windows Server 2008 R2 Standard; R 3.5.3; Rstudio 1.1.453; ggplot2 3.2.1

When I create a plot using ggplot2 that includes a continuous variable that is displayed using color, the legend appears with the range of reference values, but the color scale to the left of the numbers in the legend does not appear (it just remains white to the left of the numbers). How do I make the continuous color scale appear in the legend? Example below and I've attached the resulting plot. The color scale is missing in Rstudio and also when I export to png using png() function.

ggplot(mtcars,
       aes(x = qsec,
           y = mpg,
           colour = wt)) +
  geom_point()

image of resulting plot, which is missing the color scale in the legend

Answer 1

mlcyo's suggestion (and code in link) fixed the problem when the plot is saved to png. Thank you!