[R] plotting activity time intervals

[Martin Tomko]

Dear All,
I have interval data (for Mon-Sun, 00-24h) of an activity and would like 
to visually plot them in a matrix-like plot, where color A would be 
assigned to the activity, and color X to unspecified time usage. Note 
that the activities are not in standardised units (hours or so), but 
from startTime to endTime (in hrs:mins)
In principle it is a bar plot where multiple bars can be stacked one on 
top of another, with say the x axis representing time in a day, the y 
axis the day of the week, without gaps between the bars?
can anyone please suggest a way to plot these?
Thanks
Martin