I'm using RStudio v1.0.143 on an old Mac, and something changed one day in how RStudio handles execution of R code in the R chunks of .Rmd files.
It used to be the case where I could have the text cursor in a line, press Cmd+Enter, and it would execute the code in the console, and if the code made a plot, display it in the 'Plots' pane.
Now, a green bar on the side of the code highlights it, and it will plot/display the output below the chunk in the .Rmd file.
I have the "R Markdown" preference "Show output inline for all R Markdown documents" unchecked, and "Show equation and image previews" set to "Never". I found what should be the answer, clicking on the gear icon, and selecting "Chunk Output in Console", as seen here: 
However, when I click on the gear icon, it doesn't give me that option. Can anybody help?
