title: How to save a picture drew by image function into pdf format in Rstudio?
body: <p>I used image drawing a figure, which I want to save into pdf file. But the figure with slight gradient colour in the pdf file, but not in the .png file (if I choose the .png format). 
  Well, I need the pdf to keep the figure clear after I zoom in, so how can I save the figure drawing by the image into pdf file? </p>