add histogram to plot

[ssswzh]

Hi,

Thank you for making this great tool available in R.
I wonder if I can add a histogram (normalized expression data) through your tool. I'm not sure what type of data should I supply, I tried to transform bed to GRanges type then added it like this, but nothing happened.
ideogRam(organism = "human", assembly = 'GRCh37', chrHeight = 500) %>% add_track(GRanges(exp_dataframe))

I saw your example of adding genes to the main ideogram plot, but could you give me some examples to add a histogram as well?
Thanks in advance.

Siwen