Making subtabs in flexdashboard application - r

Question

Below you can see my test application which I built in R with flexdashboard library (pics+code)
---
title: "Test APPLICATION"
output:
flexdashboard::flex_dashboard:
orientation: columns
vertical_layout: fill
---
```{r setup, include=FALSE}
library(flexdashboard)
library(ggplot2)
library(plotly)
library(data.table)
library(tidyr)
library(dplyr)
library(tidyverse)
library(reshape)
library(knitr)
library(DT)
library(rpivotTable)
library(openintro)
library(DataCombine)
library(viridisLite)
library(eurostat)
library(tidyquant)
library(scales)
library(OECD)
library(lubridate)
library(wbstats)
library(OECD)
require(scales)
library(ggplot2)
library(dplyr)
library(openintro)
library(DataCombine)
library(viridisLite)
library(eurostat)
library(tidyquant)
library(scales)
library(readxl)
library(dplyr)
library(zoo)
library(rsconnect)
library(tidyverse)
library(tidyr)
library(data.table)
data=mtcars
data$cyl <- as.factor(data$cyl)
```
# Intro {.sidebar}
Testing application
PART 1
=======================================================================
Row {.tabset}
-----------------------------------------------------------------------
### Notes
**DIFFERENT TYPES OF CARS**
- Names:
- Consumption:
- Number of cylinders
### Tab 1
```{r,eval=TRUE}
rpivotTable(
data,
aggregatorName = "Integer Sum",
vals="mpg",
cols = "Car Parametars",
width="150%",
height="400px",
rendererName = "Table", buttons=c('copy','csv','excel','pdf','print'))
```
PART 2
=======================================================================
Row
-----------------------------------------------------------------------
### Cars 1
```{r,eval=TRUE}
fig<-qplot(x = data$wt, y = data$mpg)
fig
```
### Cars 2
```{r,eval=TRUE}
fig<-qplot(data$cyl, geom = "bar")
fig
```
Row {.tabset}
-----------------------------------------------------------------------
### Cars 3
```{r,eval=TRUE}
fig<-ggplot(data, aes(x=hp, y=mpg, color=cyl, shape=cyl)) +
geom_point(size=3)
fig
```
### Cars 4
```{r,eval=TRUE}
plt <-ggplot(data, aes(x=hp, y=mpg, color=cyl, shape=cyl)) +
geom_point(size=3) +
geom_smooth(method="lm", aes(fill=cyl))
plt + facet_wrap(~cyl)
```
Now I want to divide Tab 1. into two separate subtabs, Tab 1a and Tab 1b. Those subtabs need to be placed below Tab 1.
Tab 1 is the existing table from Tab 1, while Tab 1B is the graphs from Part 2.
After adding these tabs final output must be as in the picture below.
So can anybody help me with how to solve this problem?