[R] knitr error for small text in pdf (bug)

[Marc Girondot]

Let try this minimal Rmarkdown file

---
title: "cex in Rmarkdown"
output: word_document
---

```{r}
knitr::opts_chunk$set(dev='pdf')
```


```{r}
plot((0:160)/4, 0:160, type="n")
text(x=20, y=70, labels =expression(alpha), cex=1e-7)
```

When knitr-red from Rstudio (with r 3.6.1 on MacosX with knitr 1.24), it 
produced an error with this message ("Information of metric non 
available for this family") [translation from French because of my 
system configuration]

However,

text(x=20, y=70, labels =expression(alpha), cex=0)

and

text(x=20, y=70, labels =expression(alpha), cex=0.1)

work.

Also text(x=20, y=70, labels ="a", cex=1e-7) works.

The error does not occur when dev='png' is used but it occurs also for 
dev='postscript'

I don't know where I should report the bug.

Thanks