center googleVis in rmarkdown slidy?

Question

I would like to center, horizontally and vertically, the graphs generated with googleVis.

Right now I have this code:

---
title: "Untitled"
output: slidy_presentation
---

## Slide with googleVis

```{r, echo=F}
suppressPackageStartupMessages(library(googleVis))
op <- options(gvis.plot.tag="chart")
```



```{r results='asis', tidy=TRUE, echo=F}
Geo=gvisGeoChart(Exports, locationvar="Country", 
                 colorvar="Profit",
                 options=list(projection="kavrayskiy-vii", 
                              width = 1024,
                              keepAspectRatio=T))
cat("<center>")
print(Geo, "chart")
cat("</center>")

```

and this on my css

img.center {
        position: absolute;
        top: 50%;
        left: 50%;
        margin-right: -50%;
        transform: translate(-50%, -50%) 
}

The image is centered horizontally but not vertically.

What am I doing wrong?

Thanks!