I am plotting by Gviz pacckage. I need to get a barplot for a range, for example, from chr1:1-100, with value/score 40, I will need a bar with the hight of 40. It works by choosing type='histogram' when the value is positive. It also works when there are scores mixed with positive and negative values. But when there are all negative scores, the base line changes to the most negative value, for example chr1:1-100 with value -20, and chr1:200-300 with value -50, the base line is -50, the bar is from -50 to -20, and from -50 to 0 respectively. While I need them still from 0 to -20, and from 0 to -50. 

Any suggestion?

You can set the y-axis limits.

library(Gviz)
dt <- DataTrack(GRanges("chr1", IRanges(1:100,1:100), value=sample(-c(20,50), 100, replace=T)))
plotTracks(dt, type="histogram", ylim=c(-50,0))