I am trying to produce a plot with histogram and scatter plot in just one plot using a secondary axis. In detail, here is an example data:

```
#generate example data

set.seed(1)
a <- rnorm(200,mean=500,sd=35)
data <- data.frame(a = a,
                   b = rnorm(200, mean=10, sd=2),
                   c = c(rep(1,100), rep(0,100)))

# produce a histogram of data$a
hist(a, prob=TRUE, col="grey")

#add a density line
lines(density(a), col="blue", lwd=2)

#scatter plot 
plot(data$a,data$b,col=ifelse(data$c==1,"red","black"))

```

[![enter image description here](https://i.stack.imgur.com/35xuP.png)](https://i.stack.imgur.com/35xuP.png)
[![enter image description here](https://i.stack.imgur.com/aCsoH.png)](https://i.stack.imgur.com/aCsoH.png)

What I want to do is to combine the histogram and scatter plot together. This implies my x-axis will be `data$a`, my primary y-axis is the `frequency/density` for the histogram and my secondary y-axis is `data$b`.