Not quite sure what you want to achieve. If I knit the following Rmd file in the RStudio environment the result is a pdf file and a tex file with preamble. You can remove the preamble from the tex file.
---
# title: "Produce LaTeX from Rmarkdown"
# author: "awellis"
# date: "4/23/2020"
graphics: yes
output:
pdf_document:
keep_tex: true
---
## My plot
```{r sinus-plot, echo=FALSE}
plot(sin(seq(-pi, pi, length.out = 100)))
```
Regards Han