multiple (R) plotly figures generated in a rmarkdown (knitr chunk) document

I try to create multiple plotly figures in a Rmarkdown document using loop or lapply.

The R script:

require(plotly)
data(iris)
b <- lapply(setdiff(names(iris), c("Sepal.Length","Species")),
            function(x) {
              plot_ly(iris, 
                      x = iris[["Sepal.Length"]],
                      y = iris[[x]], 
                      mode = "markers")
            })
print(b)

works well, but it fails when included in a knitr chunk:

---
output: html_document
---

```{r,results='asis'}
require(plotly)
data(iris)
b <- lapply(setdiff(names(iris), c("Sepal.Length","Species")),
            function(x) {
              plot_ly(iris, 
                      x = iris[["Sepal.Length"]],
                      y = iris[[x]], 
                      mode = "markers")
            })
print(b)
```

I tried replacing print(b) with a combination of lapplyeval and parse but only the last figure was displayed.

I suspect a scoping/environment issue but I don't find any solution.