This does not seem like a Rmarkdown issue, but rather one of learning to use ggplot2. You should check out the data visualization chapter of R for data science as a good starting point
You can assign the ggplot as an object in the Rmd document (or even in a separate script you source from the Rmd file, as long as it is in the same R environment)
Sometimes I find it advantageous to make a big "init" chunk of code at the beginning of the Rmd that does all the work, and then use the results later on in text.
This happens mostly when I have the code already done from an exploratory phase, and want to cobble together a quick & dirty report without rewriting my code. Not the most elegant solution, but it works when time is of an issue.