Hello,

I would like to add different vertical lines with respect to each plot output (essentially with respect to different mcmc parameters) from *bayesplot*’s *mcmc_hist*. How would I do this?

For example, suppose I have some mcmc output that is a 3-dimensional array (2000 iterations, 4 chains, 4 parameters)

```
w_array
```

and suppose I have these target parameters were used to generate the sample/data for my MCMC model

```
w_target <- c(0.14, 0.34, 0.41, 0.11)
```

Then, suppose I made a 2x2 histogram using *bayesplot*’s *mcmc_hist* function resulting in 4 plots for my 4 parameters:

```
mcmc_hist(w_array) # note this will plot a histogram of mcmc results with merged chains
```

How do I add vertical lines, each representing different values of *w_target*, with respect to each plot (with respect to each parameter)? I would like a 0.14 vertical line for the first plot, 0.34 vertical line for the second plot, 0.41 vertical line for the third plot, 0.11 vertical line for the fourth plot.

Obviously, if it was just one plot, I could just *geom_vline(xintercept=#)* and be done. I have a feeling this involves the usage of the *facet_args* argument in *mcmc_hist*, but I’m completely stumped. Later, I would also like to add numerous horizontal lines to my numerous trace plots or extend the code logic to other types of *bayesplots*

Best,

Michael