white space between title and plot in ioslides

I am using the default theme of ioslides for a presentation, and I have difficulties with arranging a plot on the slide.

I use the following code for the respective slide:

```{r crint1, fig.width = 7, fig.height = 5.5, fig.retina = NULL, fig.align="center"}
options(warn=-1)
load("path_to_data.RData")
ord <- order(cor.table$COR)
barplot(cor.table$COR[ord], names.arg=cor.table$group[ord],horiz=T
        , cex.name=.6, border=F, space=.3, col="gray30", 
        xlab="Correlation Coefficient", las=1)
  grid(NULL, NA, col="white", lty="solid", lwd=0.7)
    abline(v=mean(cor.table$COR), lwd=2, lty=2, col="red")
      text(mean(cor.table$COR), 1, "Average", cex=.8, col="red", pos=4)```

Which results in this:

Which is okay, but I would like the plot to start right below the title and extend more on the slide, so the names of the rows are readable and don't overlap.

I tried to play around with fig.height and fig.width, but they only extend the size of the plot and do not change the starting coordinates (which makes the plot extend beyond the slide).

Is there any way to reduce the white space below the title and use that margin to extend the plot? I assume I have to edit the CSS for that particular slide, I am just not sure how. Any help is appreciated.