This page contains miscellaneous questions that I have been asked about visreg, with answers posted here so that others can read them too.

## Q: In `visreg2d`

, is it possible to superimpose data points onto the plot?

Yes; it’s probably easiest to do this using `ggplot2`

to render the surface:

```
fit <- lm(Ozone ~ Solar.R + Wind + Temp + I(Wind^2) + I(Temp^2) + I(Wind*Temp)+I(Wind*Temp^2) +
I(Temp*Wind^2) + I(Temp^2*Wind^2), data=airquality)
visreg2d(fit, x="Wind", y="Temp", plot.type="gg") +
geom_point(aes(Wind, Temp), data=subset(airquality, !is.na(Ozone)), col='gray50')
```

You can do it with base R graphics too, but it’s kind of complicated because of how `filled.contour()`

works and the way in which it actually creates two plots, one main plot and one plot for the legend:

```
p <- quote({
axis(1, at = mx, labels = lx)
axis(2, at = my, labels = ly)
with(airquality, points(Wind, Temp, pch=19))
})
visreg2d(fit, x="Wind", y="Temp", plot.type="image", plot.axes=p)
```

Doing this through the axes is admittedly very weird – it’s not my idea; see `?filled.contour`

.

## Q: Does `visreg`

work with `gamm`

(mgcv) or `gamm4`

objects?

Yes, although it takes a minor workaround since these objects have an unusual structure. First, let’s simulate some data and fit a model (I’m just using a `tibble`

here for convenience, it isn’t relevant to the question):

```
library(mgcv)
library(tibble)
n <- 20
Data <- tibble(
ID = factor(rep(LETTERS[1:n], each=5)),
x = rnorm(n*5),
lp = rnorm(n)[as.numeric(ID)] + x^2 - 1,
p = binomial()$linkinv(lp),
y = rbinom(n*5, 1, p)
)
fit <- gamm(y~s(x), data=Data, dist='binomial', random=list(ID=~1))
```

Now, `fit$gam`

does not include the call (i.e., `fit$gam$call`

is `NULL`

), which means `visreg`

won’t be able to find the data:

```
visreg(fit$gam, 'x')
# Error in FUN(X[[i]], ...): object 'y' not found
```

So you have to include it manually:

```
fit$gam$data <- Data
visreg(fit$gam, 'x')
```

## Q: I’m getting the following error message: `Error in UseMethod("family") : no applicable method for 'family' applied to an object of class XXXX`

.

When you specify `scale='response'`

, you’re telling `visreg`

to automatically find the transformation mapping the linear predictor to the scale of the response. It can only do that if the fitted model object provides that transformation through the `family()`

method. Many R models (e.g., `glm`

) do this, but some (e.g., the `glmmADMB`

package, `gamm`

from the `mgcv`

package) don’t. If you’re working with one of these models, you’ll have to supply that transformation yourself. For example, with the model from the previous question, this code fails:

```
visreg(fit$gam, 'x', scale='response')
# Error in UseMethod("family"): no applicable method for 'family' applied to an object of class "gam"
```

Since this is a binomial model with a logistic link, `binomial()$linkinv`

provides the inverse transformation:

## Q: Can I have the response variable on the horizontal axis instead of the vertical axis?

You can achieve this via `ggplot2::coord_flip`

: