I am plotting a set of data that I'd like to layer on top of each other. This is sequencing read depth from two samples. Here a snippet of my data

GRanges object with 3828 ranges and 2 metadata columns:
         seqnames                 ranges strand |    depth1     depth2
            <Rle>              <IRanges>  <Rle> | <integer> <integer>
     [1]     chr5 [171387115, 171387116]      * |         6        10
     [2]     chr5 [171387116, 171387117]      * |         5        10
     [3]     chr5 [171387117, 171387118]      * |         6        10
     [4]     chr5 [171387118, 171387119]      * |         6        11
     [5]     chr5 [171387119, 171387120]      * |         5         9

When I plot this data with the histogram function using

data_track <- DataTrack(data_grange, name = " ", col = c("black","grey"))
plotTracks(data_track, type = "histogram", showId = TRUE, c("depth1","depth2"))

The histograms are stacked one on top of the other. As such

I'd like to have them layered instead to show the difference between the two samples. Stacking is in this instance is very hard to interpret. 

Thanks for any an all advice.