感谢this link。使用标题有助于使情节居中。这只是一个建议，而不是对你的问题的明确回答。

---
 title: "Test"
 author: "My Name"
 output: pdf_document
 header-includes:
   - \usepackage{subfig}
---  

```{r echo =FALSE, results=FALSE}
captions <- c("Caption 1",
              "Caption 2", 
              "Caption 3",
              "Caption 4")

library(ggplot2)
 # line chart
 p1<- ggplot(pressure, aes(x = temperature, y = pressure)) +
         geom_line()

 # step chart
 p2<-ggplot(pressure, aes(x = temperature, y = pressure)) +
         geom_step()

 # line chart with points
 p3<-ggplot(pressure, aes(x = temperature, y = pressure)) +
         geom_line() +
         geom_point()

 # line chart
 p4<-ggplot(pressure, aes(x = temperature, y = pressure)) +
         geom_line()

 p1
 p2
 p3
 p4

我相信这种行为源于LaTeX将数字作为浮点数处理的事实。

...
output: 
    pdf_document:
        # improve float placement
        # see https://bookdown.org/yihui/rmarkdown-cookbook/figure-placement.html
        extra_dependencies: ["flafter"]

或

...
  output: 
    pdf_document:
        # Another way to try and control float placement
        extra_dependencies: ["float"]

请注意，间距关系到knitr如何解释YAML。
另一种选择是使数字小一点，看看这是否有助于浮动放置。