title: Reading geo-tiff file using raster() ggplot fails with no error message. What causes this error?
body: <p>I'm attempting to learn how to import, display, and generally handle geo-tiff files in R-Studio and notebook. When I run the code I get no errors. The plot is not displayed but entering the plot name in the console gives an error. It's as if an error is detected, the plot is still created, but no error is reported either by running the chunks or by running 'knit'.  </p>
  
  <blockquote>
    <p><code>fimage_plot    Error: Discrete value supplied to continuous scale</code></p>
  </blockquote>
  
  <p>My code chunk:</p>
  
  <pre><code>rlist &lt;- list.files(tiffTestFldrOrig, pattern="tif$",
                  full.names=TRUE)
  for(tiff_path_nane in rlist) {
    fimage &lt;- raster(tiff_path_nane)
    fill_col_name = names(fimage)
    fimage_df &lt;- as.data.frame(fimage, xy = TRUE)
  
    fimage_plot &lt;- ggplot() +
      geom_raster(data = fimage_df, aes(x=x, y=y, 
                fill = fill_col_name)) +
      scale_fill_continuous(type = "gradient") +
      coord_quickmap()
  
    fimage_plot # no plot displayed, no error
    break() # until error corrected
  }
  </code></pre>
  
  <p>I've tried google, searching on various <code>scale_fill_discete</code>, <code>scale_fill_continous</code>, etc. to no avail.</p>
  
  <p>BTW my x &amp; y data are UTM with the third column 16 bit integer values representing temperatures of a wildfire. </p>