Cem,
It seems that the way the bar graph works relies on there being more than one point to 'close' the bars. Because you are only plotting one point per plot the bar can't be 'closed'. A way round this might be to make each plot have two points spanning over one interval and let each plot overlap the next.
eg. Plot 1 (1,1 ; 2,1)
Plot 2 (2,5 ; 3,5)
Plot 3 (3,4 ; 4,4)
.
.
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I haven't tried this myself but I think it should work.
John